An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.

Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a short-arm cross or a kite. These shapes allow for the creation of the envelope structure by folding the sheet sides around a central rectangular area. In this manner, a rectangle-faced enclosure is formed with an arrangement of four flaps on the reverse side.

Envelope - Boonville Address-000
Front of an envelope mailed in the U.S. in 1906, with a postage stamp and address
Envelope - Boonville Address-002
Back of the above envelope, showing an additional receiving office postmark
Tyvek 1056D Envelope 13032009849
DL Tyvek envelope


Patent drawing of Americus Callahan's windowed envelope

When the folding sequence is such that the last flap to be closed is on a short side it is referred to in commercial envelope manufacture as a pocket - a format frequently employed in the packaging of small quantities of seeds. Although in principle the flaps can be held in place by securing the topmost flap at a single point (for example with a wax seal), generally they are pasted or gummed together at the overlaps. They are most commonly used for enclosing and sending mail (letters) through a prepaid-postage postal system.

Window envelopes have a hole cut in the front side that allows the paper within to be seen.[1] They are generally arranged so that the receiving address printed on the letter is visible, saving the sender from having to duplicate the address on the envelope itself. The window is normally covered with a transparent or translucent film to protect the letter inside, as was first designed by Americus F. Callahan in 1901 and patented the following year.[2] In some cases, shortages of materials or the need to economize resulted in envelopes that had no film covering the window. One innovative process, invented in Europe about 1905, involved using hot oil to saturate the area of the envelope where the address would appear. The treated area became sufficiently translucent for the address to be readable. As of 2009 there is no international standard for window envelopes, but some countries, including Germany and the United Kingdom, have national standards.[3]

An aerogram is related to a lettersheet, both being designed to have writing on the inside to minimize the weight. Any handmade envelope is effectively a lettersheet because prior to the folding stage it offers the opportunity for writing a message on that area of the sheet that after folding becomes the inside of the face of the envelope.

A Japanese funeral envelope used for offering condolence money. The white and black cords represent death. Similar-looking envelopes with red and silver cords are used for weddings.

The "envelope" used to launch the Penny Post component of the British postal reforms of 1840 by Sir Rowland Hill and the invention of the postage stamp, was a lozenge-shaped lettersheet known as a Mulready.[4] If desired, a separate letter could be enclosed with postage remaining at one penny provided the combined weight did not exceed half an ounce (14 grams). This was a legacy of the previous system of calculating postage, which partly depended on the number of sheets of paper used.

During the U.S. Civil War those in the Confederate States Army occasionally used envelopes made from wallpaper, due to financial hardship.

A "return envelope" is a pre-addressed, smaller envelope included as the contents of a larger envelope and can be used for courtesy reply mail, metered reply mail, or freepost (business reply mail). Some envelopes are designed to be reused as the return envelope, saving the expense of including a return envelope in the contents of the original envelope. The direct mail industry makes extensive use of return envelopes as a response mechanism.

Up until 1840, all envelopes were handmade, each being individually cut to the appropriate shape out of an individual rectangular sheet. In that year George Wilson in the United Kingdom patented the method of tessellating (tiling) a number of envelope patterns across and down a large sheet, thereby reducing the overall amount of waste produced per envelope when they were cut out. In 1845 Edwin Hill and Warren de la Rue obtained a patent for a steam-driven machine that not only cut out the envelope shapes but creased and folded them as well. (Mechanised gumming had yet to be devised.) The convenience of the sheets ready cut to shape popularized the use of machine-made envelopes, and the economic significance of the factories that had produced handmade envelopes gradually diminished.

As envelopes are made of paper, they are intrinsically amenable to embellishment with additional graphics and text over and above the necessary postal markings. This is a feature that the direct mail industry has long taken advantage of—and more recently the Mail Art movement. Custom printed envelopes has also become an increasingly popular marketing method for small business.

Most of the over 400 billion envelopes of all sizes made worldwide are machine-made.


International standard sizes

International standard ISO 269 defined (It has since become a withdrawn standard.) several standard envelope sizes, which are designed for use with ISO 216 standard paper sizes:

Format Dimensions (mm) Dimensions (in) Suitable for content format
DL 110 × 220 4.33 × 8.66 13 A4
C7 81 × 114 3.2 × 4.5 A7 (or ​12 A6)
C7/C6 81 × 162 3.19 × 6.4 13 A5
C6 114 × 162 4.5 × 6.4 A6 (or ​12 A5 or ​14 A4)
C6/C5 114 × 229 4.5 × 9 13 A4
C5 162 × 229 6.4 × 9 A5 (or ​12 A4)
C4 229 × 324 9.0 × 12.8 A4
C3 324 × 458 12.8 × 18 A3
B6 125 × 176 4.9 × 6.9 C6
B5 176 × 250 6.9 × 9.8 C5
B4 250 × 353 9.8 × 13.9 C4
E4 280 × 400 11 × 15.75 B4

The German standard DIN 678 defines a similar list of envelope formats.

North American sizes

There are dozens of sizes of envelopes available.

The designations such as "A2" do not correspond to ISO paper sizes. (Often, North American paper jobbers and printers will insert a hyphen to distinguish from ISO sizes, thus: A-2.)

Format Dimensions (in) Dimensions (mm) Ratio
A2 4 38 × ​5 34 111.1 × 146.1 132%
A6 4 34 × ​6 12 120.7 × 165.1 137%
A7 5 14 × ​7 14 133.4 × 184.2 138%
A8 5 12 × ​8 18 139.7 × 206.4 148%
A9 5 34 × ​8 34 146.1 × 222.3 152%
A10 6 × ​9 12 152.4 × 241.3 158%
No. ​6 34 3 58 × ​6 12 92.1 × 165.1 179%
No. ​7 34 (Monarch) 3 78 × ​7 12 98.4 × 190.5 194%
No. 9 3 78 × ​8 78 98.4 × 225.4 229%
No. 10 4 18 × ​9 12 104.8 × 241.3 230%
No. 11 4 12 × ​10 38 114.3 × 263.5 231%
No. 12 4 34 × 11 120.7 × 279.4 232%
No. 14 5 × ​11 12 127.0 × 292.1 230%

The No. 10 envelope is the standard business envelope size in the United States.[5]

Envelopes accepted by the U.S. Postal Service for mailing at the price of a letter must be:

  • Rectangular
  • At least ​3 12 inches high × 5 inches long × 0.007 inch thick.
  • No more than ​6 18 inches high × ​11 12 inches long × ​14 inch thick.[6]
  • Letters that have a length-to-height aspect ratio of less than 1.3 or more than 2.5 are classified as "non-machinable" by the USPS and may cost more to mail.[7]

United Kingdom sizes

Check out Central Mailing Services infographic here for the general sizes of envelopes in the United Kingdom.


History of envelopes

Employement contract IMG 0074
Tablet and its sealed envelope: employment contract. Girsu, Sumer, circa 2037 BC. Terra cotta. Museum of Fine Arts of Lyon.
Red envelopes are an example of paper envelopes. They are used for monetary gifts.

The first known envelope was nothing like the paper envelope we know of today. It can be dated back to around 3500 to 3200 BC in the ancient Middle East. Hollow, clay spheres were molded around financial tokens and used in private transactions. The two people who discovered these first envelopes were Jacques de Morgan, in 1901, and Roland de Mecquenem, in 1907.

Paper envelopes were developed in China, where paper was invented by 2nd century BC.[8] Paper envelopes, known as chih poh, were used to store gifts of money. In the Southern Song dynasty, the Chinese imperial court used paper envelopes to distribute monetary gifts to government officials.[9]

Prior to 1845, hand-made envelopes were all that were available for use, both commercial and domestic. In 1845, Edwin Hill and Warren De La Rue were granted a British patent for the first envelope-making machine.[10]

The "envelopes" produced by the Hill/De La Rue machine were not as we know them today. They were flat diamond, lozenge (or rhombus)-shaped sheets or "blanks" that had been precut to shape before being fed to the machine for creasing and made ready for folding to form a rectangular enclosure. The edges of the overlapping flaps treated with a paste or adhesive and the method of securing the envelope or wrapper was a user choice. The symmetrical flap arrangement meant that it could be held together with a single wax seal at the apex of the topmost flap. (That the flaps of an envelope can be held together by applying a seal at a single point is a classic design feature of an envelope.)

Nearly 50 years passed before a commercially successful machine for producing pre-gummed envelopes effectively as we know them today appeared.

The origin of the use of the diamond shape for envelopes is debated. However, as an alternative to simply wrapping a sheet of paper around a folded letter or an invitation and sealing the edges, it is a tidy and ostensibly paper-efficient way of producing a rectangular-faced envelope. Where the claim to be paper-efficient fails is a consequence of paper manufacturers normally making paper available in rectangular sheets, because the largest size of envelope that can be realised by cutting out a diamond or any other shape which yields an envelope with symmetrical flaps is smaller than the largest that can be made from that sheet simply by folding.

Envelope - Wood Food Company-000
Envelope with advertising from 1905 used in the U.S.

The folded diamond-shaped sheet (or "blank") was in use at the beginning of the 19th century as a novelty wrapper for invitations and letters among the proportion of the population that had the time to sit and cut them out and were affluent enough not to bother about the waste offcuts. Their use first became widespread in the UK when the British government took monopoly control of postal services and tasked Rowland Hill with its introduction. The new service was launched in May 1840 with a postage-paid machine-printed illustrated (or pictorial) version of the wrapper and the much-celebrated first adhesive postage stamp: the Penny Black- for the production of which the Jacob Perkins printing process was used to deter counterfeiting and forgery. The wrappers were printed and sold as a sheet of 12, with cutting the purchaser's task. Known as Mulready stationery, because the illustration was created by the respected artist William Mulready, the envelopes were withdrawn when the illustration was ridiculed and lampooned. Nevertheless, the public apparently saw the convenience of the wrappers being available ready-shaped, and it must have been obvious that with the stamp available totally plain versions of the wrapper could be produced and postage prepaid by purchasing a stamp and affixing it to the wrapper once folded and secured. In this way although the postage-prepaid printed pictorial version died ignominiously, the diamond-shaped wrapper acquired de facto official status and became readily available to the public notwithstanding the time taken to cut them out and the waste generated. With the issuing of the stamps and the operation and control of the service (which is a communications medium) in government hands the British model spread around the world and the diamond-shaped wrapper went with it.

Hill also installed his brother Edwin as The Controller of Stamps, and it was he with his partner Warren De La Rue who patented the machine for mass-producing the diamond-shaped sheets for conversion to envelopes in 1845. Today, envelope-making machine manufacture is a long- and well-established international industry, and blanks are produced with a short-arm-cross shape and a kite shape as well as diamond shape. (The short-arm-cross style is mostly encountered in "pocket" envelopes i.e. envelopes with the closing flap on a short side. The more common style, with the closing flap on a long side, are sometimes referred to as "standard" or "wallet" style for purposes of differentiation.)

Blythe House Envelope-making machines 1930s
Envelope-making machines at the Post Office Savings Bank, Blythe House, West Kensington, London
Machine Envelope Printer
Machine Envelope Printer was one of the machine presses at the Bulaq Press. It present now in Bibliotheca Alexandrina

The most famous paper-making machine was the Fourdrinier machine. The process involves taking processed pulp stock and converting it to a continuous web which is gathered as a reel. Subsequently, the reel is guillotined edge to edge to create a large number of properly rectangular sheets because ever since the invention of Gutenberg's press paper has been closely associated with printing.

To this day, all other mechanical printing and duplicating equipments devised in the meantime, including the typewriter (which was used up to the 1990s for addressing envelopes), have been primarily designed to process rectangular sheets. Hence the large sheets are in turn are guillotined down to the sizes of rectangular sheet commonly used in the commercial printing industry, and nowadays to the sizes commonly used as feed-stock in office-grade computer printers, copiers and duplicators (mainly ISO, A4 and US Letter).

Using any mechanical printing equipment to print on envelopes, which although rectangular, are in fact folded sheets with differing thicknesses across their surfaces, calls for skill and attention on the part of the operator. In commercial printing the task of printing on machine-made envelopes is referred to as "overprinting" and is usually confined to the front of the envelope. If printing is required on all four flaps as well as the front, the process is referred to as "printing on the flat". Eye-catching illustrated envelopes or pictorial envelopes, the origins of which as an artistic genre can be attributed to the Mulready stationery – and which was printed in this way - are used extensively for direct mail. In this respect, direct mail envelopes have a shared history with propaganda envelopes (or "covers") as they are called by philatelists.

Present and future state of envelopes

At the end of the 20th century, in 1998, the digital printing revolution delivered another benefit for small businesses when the U.S. Postal Service became the first postal authority to approve the introduction of a system of applying to an envelope in the printer bin of a PC sheet printer a digital frank or stamp delivered via the Internet. With this innovative alternative to an adhesive-backed postage stamp as the basis for an Electronic Stamp Distribution (ESD) service, a business envelope could be produced in-house, addressed and customized with advertising information on the face, and ready to be mailed.

The fortunes of the commercial envelope manufacturing industry and the postal service go hand in hand, and both link to the printing industry and the mechanized envelope processing industry producing equipment such as franking and addressing machines. They are all four symbiotic: technological developments affecting one obviously ricochet through the others: addressing machines print addresses, postage stamps are a print product, franking machines imprint a frank on an envelope. If fewer envelopes are required; fewer stamps are required; fewer franking machines are required and fewer addressing machines are required. For example, the advent and adoption of information-based indicia (IBI) (commonly referred to as digitally-encoded electronic stamps or digital indicia) by the US Postal Service in 1998 caused widespread consternation in the franking machine industry, as their equipments were effectively rendered obsolescent and resulted in a flurry of lawsuits involving Pitney Bowes among others. The advent of e-mail in the late 1990s appeared to offer a substantial threat to the postal service. By 2008 letter-post service operators were reporting significantly smaller volumes of letter-post, specifically stamped envelopes, which they attributed mainly to replacement by e-mail. Although a corresponding reduction in the volume of envelopes required would have been expected, no such decrease was reported as widely as the reduction in letter-post volumes.

Although, with regards to e-mail developments, there is a substantial threat of "technology replacing tradition". This is offset by the equal reasoning that the Universal Postal Union is an international specialised agency of the United Nations, and a source of revenue for the government. Consequently, any deterioration of domestic and international postal services attended by loss of revenue is a matter of governmental concern.

Types of envelopes

Windowed envelopes

DIN lang - Letter Window
Windowed envelope

A windowed envelope is an envelope with a plastic or glassine window in it. The plastic in these envelopes creates problems in paper recycling. Consumers who do not want to go through the trouble of ripping out the plastic window should put the envelope in a trash bag after use.

Security envelopes

Security envelopes have special tamper-resistant and tamper-evident features. They are used for high value products and documents as well as for evidence for legal proceedings.

Some security envelopes have a patterned tint printed on the inside, which makes it difficult to read the contents. Various patterns exist.[11]


Some envelopes are available for full size documents. Some carriers have large mailing envelopes for their express services. Other similar envelopes are available at stationery supply locations.

These mailers usually have an opening on an end with a flap that can be attached by gummed adhesive, integral pressure-sensitive adhesive, adhesive tape, or security tape. Construction is usually:

Padded mailers

Shipping envelopes can have padding to provide stiffness and some degree of cushioning. The padding can be ground newsprint, plastic foam sheets, or bubble packing.

Inter-office envelopes

Various U.S. Federal Government offices use Standard Form (SF) 65 Government Messenger Envelopes for inter-office mail delivery. These envelopes are typically light brown in color and un-sealed with string-tied closure method and an array of holes throughout both sides such that it is somewhat visible what the envelope contains. Other colloquial names for this envelope include "Holey Joe" and "Shotgun" envelope due to the holey nature of the envelope. Address method is unique in that these envelopes are re-usable and the previous address is crossed out thoroughly and the new addressee (name, building, room, and mailstop) is written in the next available box. Although still in use, SF-65 is no longer listed on the United States Office of Personnel Management website list of standard forms, which may indicate new envelopes are no longer being printed. [12]

See also


  1. ^ "History of Envelopes". BE. Retrieved 27 December 2014. Window envelopes have a small plastic pane that fits an address printed onto the letter inside. Windowed envelopes soon became the standard for business envelopes, as they reduce the time and cost required to send mail while still ensuring it gets delivered to its intended destination.
  2. ^ "US 701839 A". Retrieved 27 December 2014.
  3. ^ A software company's information on US and ISO international standard envelope styles and sizes
  4. ^ "Mulready stationery: Lettersheets and envelopes". The Queen's Own: Stamps That Changed the World. National Postal Museum. Retrieved 2006-09-25.
  5. ^ "Envelope Size Chart - Help understanding envelope sizes". PaperPapers.com. 2018. Retrieved 2018-02-13.
  6. ^ "Sizes for Letters". USPS. 2016. Retrieved 2016-12-24.
  7. ^ "Physical Standards for Commercial Letters and Postcards" (PDF). USPS. 2018. Retrieved 2018-02-13.
  8. ^ Tsien, Tsuen-Hsuin (1985). "Paper and Printing". Joseph Needham, Science and Civilisation in China, Chemistry and Chemical Technology. 5 part 1. Cambridge University Press: 38.
  9. ^ Joseph Needham (1985). Science and Civilisation in China: Paper and Printing. Cambridge University Press. p. 122. ISBN 978-0-521-08690-5. In the Southern Sung dynasty, gift money for bestowing upon officials by the imperial court was wrapped in paper envelopes (chih pao)
  10. ^ "The Heroic Age". Making the Modern World. Retrieved 2012-11-06.
  11. ^ See Security Patterns by Joseph King for a selection of security patterns from around the world
  12. ^ See NIH.GOV Inter-Office Communications Mail Guide for an example of usage instructions

External links


An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from large gasbags filled with a lifting gas that is less dense than the surrounding air.

In early dirigibles, the lifting gas used was hydrogen, due to its high lifting capacity and ready availability. Helium gas has almost the same lifting capacity and is not flammable, unlike hydrogen, but is rare and relatively expensive. Significant amounts were first discovered in the United States and for a while helium was only used for airships in that country. Most airships built since the 1960s have used helium, though some have used hot air.The envelope of an airship may form a single gasbag, or may contain a number of internal gas-filled cells. An airship also has engines, crew, and optionally also payload accommodation, typically housed in one or more "gondolas" suspended below the envelope.

The main types of airship are non-rigid, semi-rigid, and rigid. Non-rigid airships, often called "blimps", rely on internal pressure to maintain their shape. Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of supporting structure, such as a fixed keel, attached to it. Rigid airships have an outer structural framework that maintains the shape and carries all structural loads, while the lifting gas is contained in one or more internal gasbags or cells. Rigid airships were first flown by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded. As a result, rigid airships are often called zeppelins.Airships were the first aircraft capable of controlled powered flight, and were most commonly used before the 1940s; their use decreased over time as their capabilities were surpassed by those of aeroplanes. Their decline was accelerated by a series of high-profile accidents, including the 1930 crash and burning of British R101 in France, the 1933 and 1935 storm-related crashes of the twin airborne aircraft carrier U.S. Navy helium-filled rigids, the USS Akron and USS Macon respectively, and the 1937 burning of the hydrogen-filled Hindenburg. From the 1960s, helium airships have been used in applications where the ability to hover in one place for an extended period outweighs the need for speed and manoeuvrability, such as advertising, tourism, camera platforms, geological surveys, and aerial observation.

Back-of-the-envelope calculation

A back-of-the-envelope calculation is a rough calculation, typically jotted down on any available scrap of paper such as an envelope. It is more than a guess but less than an accurate calculation or mathematical proof. The defining characteristic of back-of-the-envelope calculations is the use of simplified assumptions. A similar phrase in the U.S. is "back of a napkin", also used in the business world to describe sketching out a quick, rough idea of a business or product. In British English, a similar idiom is "back of a fag packet".

Building envelope

A building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer.


A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic material of the virus.

Capsids are broadly classified according to their structure. The majority of viruses have capsids with either helical or icosahedral structure. Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity and electrostatics. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape resembles the shape of a spring, taking the space of a cylinder but not being a cylinder itself. The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins named VP1–3.Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane in the virus' host; examples include the inner nuclear membrane, the golgi membrane, and the cell's outer membrane.Once the virus has infected a cell and begins replicating itself, new capsid subunits are synthesized using the protein biosynthesis mechanism of the cell. In some viruses, including those with helical capsids and especially those with RNA genomes, the capsid proteins co-assemble with their genomes. In other viruses, especially more complex viruses with double-stranded DNA genomes, the capsid proteins assemble into empty precursor procapsids that includes a specialized portal structure at one vertex. Through this portal, viral DNA is translocated into the capsid.Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into lineages. For example, the bacteriophage PRD1, the algal virus Paramecium bursaria Chlorella virus (PBCV-1), mimivirus and the mammalian adenovirus have been placed in the same lineage, whereas tailed, double-stranded DNA bacteriophages (Caudovirales) and herpesvirus belong to a second lineage.

Cell envelope antibiotic

A cell envelope antibiotic is an antibacterial that acts primarily at the level of the cell envelope.

Examples include cycloserine, penicillin, and polymyxin B.

Cell nucleus

In cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many.

The cell nucleus contains all of the cell's genome, except for a small fraction of mitochondrial DNA, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.

Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, and a number of nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.

Circumstellar envelope

A circumstellar envelope (CSE) is a part of a star that has a roughly spherical shape and is not gravitationally bound to the star core. Usually circumstellar envelopes are formed from the dense stellar wind, or they are present before the formation of the star. Circumstellar envelopes of the old stars (Mira variables and OH/IR stars) eventually evolve into protoplanetary nebulae, and circumstellar envelopes of the young stellar objects evolve into circumstellar discs.

Common envelope

In astronomy, a common envelope (CE) is gas that contains a binary star system. The gas does not rotate at the same rate as the embedded binary system. A system with such a configuration is said to be in a common envelope phase or undergoing common envelope evolution.

During a common envelope phase the embedded binary system is subject to drag forces from the envelope which cause the separation of the two stars to decrease. The phase ends either when the envelope is ejected to leave the binary system with much smaller orbital separation, or when the two stars become sufficiently close to merge and form a single star. A common envelope phase is short-lived relative to the lifetime of the stars involved.

Evolution through a common envelope phase with ejection of the envelope can lead to the formation of a binary system composed of a compact object with a close companion. Cataclysmic variables, X-ray binaries and systems of close double white dwarfs or neutron stars are examples of systems of this type which can be explained as having undergone common envelope evolution. In all these examples there is a compact remnant (a white dwarf, neutron star or black hole) which must have been the core of a star which was much larger than the current orbital separation. If these systems have undergone common envelope evolution then their present close separation is explained. Short-period systems containing compact objects are sources of gravitational waves and Type Ia supernovae.

Predictions of the outcome of common envelope evolution are uncertain.A common envelope is sometimes confused with a contact binary. In a common envelope binary system the envelope does not generally rotate at the same rate as the embedded binary system; thus it is not constrained by the equipotential surface passing through the L2 Lagrangian point. In a contact binary system the shared envelope rotates with the binary system and fills an equipotential surface.

Convex hull

In mathematics, the convex hull or convex envelope or convex closure of a set X of points in the Euclidean plane or in a Euclidean space (or, more generally, in an affine space over the reals) is the smallest convex set that contains X. For instance, when X is a bounded subset of the plane, the convex hull may be visualized as the shape enclosed by a rubber band stretched around X.Formally, the convex hull may be defined either as the intersection of all convex sets containing X, or as the set of all convex combinations of points in X. With the latter definition, convex hulls may be extended from Euclidean spaces to arbitrary real vector spaces; they may also be generalized further, to oriented matroids.The algorithmic problem of finding the convex hull of a finite set of points in the plane or other low-dimensional Euclidean spaces is one of the fundamental problems of computational geometry.

Envelope (music)

In sound and music, an envelope describes how a sound changes over time. Envelopes may relate to elements such as amplitude (volume), a filter (frequencies), or pitch. For example, a piano key, when struck and held, creates a near-immediate initial sound, which gradually decreases in volume to zero. Envelope generators are common features of synthesizers, samplers, and other electronic musical instruments.

Flight envelope

In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft or interplanetary spacecraft refers to the capabilities of a design in terms of airspeed and load factor or atmospheric density, often simplified to altitude for Earth-borne aircraft. The term is somewhat loosely applied, and can also refer to other measurements such as manoeuvrability. When a plane is pushed, for instance by diving it at high speeds, it is said to be flown "outside the envelope", something considered rather dangerous.

Flight envelope is one of a number of related terms that are all used in a similar fashion. It is perhaps the most common term because it is the oldest, first being used in the early days of test flying. It is closely related to more modern terms known as extra power and a doghouse plot which are different ways of describing a flight envelope. In addition, the term has been widened in scope outside the field of engineering, to refer to the strict limits in which an event will take place or more generally to the predictable behaviour of a given phenomenon or situation, and hence, its "flight envelope".

Hot air balloon

A hot air balloon is a lighter-than-air aircraft consisting of a bag, called an envelope, which contains heated air. Suspended beneath is a gondola or wicker basket (in some long-distance or high-altitude balloons, a capsule), which carries passengers and a source of heat, in most cases an open flame caused by burning liquid propane. The heated air inside the envelope makes it buoyant since it has a lower density than the colder air outside the envelope. As with all aircraft, hot air balloons cannot fly beyond the atmosphere. Unlike gas balloons, the envelope does not have to be sealed at the bottom, since the air near the bottom of the envelope is at the same pressure as the surrounding air. In modern sport balloons the envelope is generally made from nylon fabric and the inlet of the balloon (closest to the burner flame) is made from a fire resistant material such as Nomex. Modern balloons have been made in all kinds of shapes, such as rocket ships and the shapes of various commercial products, though the traditional shape is used for most non-commercial, and many commercial, applications.

The hot air balloon is the first successful human-carrying flight technology. The first untethered manned hot air balloon flight was performed by Jean-François Pilâtre de Rozier and François Laurent d'Arlandes on November 21, 1783, in Paris, France, in a balloon created by the Montgolfier brothers. The first hot-air balloon flown in the Americas was launched from the Walnut Street Jail in Philadelphia on January 9, 1793 by the French aeronaut Jean Pierre Blanchard. Hot air balloons that can be propelled through the air rather than simply drifting with the wind are known as thermal airships.

Nuclear envelope

The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes which in eukaryotic cells surrounds the nucleus, which encases the genetic material.

The nuclear envelope consists of two lipid bilayer membranes, an inner nuclear membrane, and an outer nuclear membrane. The space between the membranes is called the perinuclear space. It is usually about 20–40 nm wide. The outer nuclear membrane is continuous with the endoplasmic reticulum membrane. The nuclear envelope has many nuclear pores that allow materials to move between the cytosol and the nucleus. Intermediate filaments form a lamina internally to the inner nuclear membrane, and more loosely externally to the outer nuclear membrane to give structural support to the nucleus.

Red envelope

In Chinese and other East Asian and Southeast Asian societies, a red envelope or a red packet is a monetary gift which is given during holidays or special occasions such as weddings, graduation or the birth of a baby.

Outside of China, similar customs have been adopted across parts of Southeast Asia and many other countries with a sizable ethnic Chinese population.

Security bag

A security bag is a heavy duty bag used to contain high-value products or documents or legally sensitive items. Envelopes with security features are called security envelopes as well as security bags. When used to contain items related to a crime, special evidence bags are used. Authentication of signatures and chain of custody are often required.

Structure and genome of HIV

The genome and proteins of HIV (human immunodeficiency virus) have been the subject of extensive research since the discovery of the virus in 1983. "In the search for the causative agent, it was initially believed that the virus was a form of the Human T-cell leukemia virus (HTLV), which was known at the time to affect the human immune system and cause certain leukemias. However, researchers at the Pasteur Institute in Paris isolated a previously unknown and genetically distinct retrovirus in patients with AIDS which was later named HIV." Each virion comprises a viral envelope and associated matrix enclosing a capsid, which itself encloses two copies of the single-stranded RNA genome and several enzymes. The discovery of the virus itself occurred two years following the report of the first major cases of AIDS-associated illnesses.


A synthesizer or synthesiser (often abbreviated to synth) is an electronic musical instrument that generates audio signals that may be converted to sound. Synthesizers may imitate traditional musical instruments such as piano, flute, vocals, or natural sounds such as ocean waves; or generate novel electronic timbres. They are often played with a musical keyboard, but they can be controlled via a variety of other devices, including music sequencers, instrument controllers, fingerboards, guitar synthesizers, wind controllers, and electronic drums. Synthesizers without built-in controllers are often called sound modules, and are controlled via USB, MIDI or CV/gate using a controller device, often a MIDI keyboard or other controller.

Synthesizers use various methods to generate electronic signals (sounds). Among the most popular waveform synthesis techniques are subtractive synthesis, additive synthesis, wavetable synthesis, frequency modulation synthesis, phase distortion synthesis, physical modeling synthesis and sample-based synthesis.

Synthesizers were first used in pop music in the 1960s. In the late 1970s, synths were used in progressive rock, pop and disco. In the 1980s, the invention of the relatively inexpensive Yamaha DX7 synth made digital synthesizers widely available. 1980s pop and dance music often made heavy use of synthesizers. In the 2010s, synthesizers are used in many genres, such as pop, hip hop, metal, rock and dance. Contemporary classical music composers from the 20th and 21st century write compositions for synthesizer.


In music, timbre ( TAM-bər, also known as tone color or tone quality from psychoacoustics) is the perceived sound quality of a musical note, sound or tone. Timbre distinguishes different types of sound production, such as choir voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. It also enables listeners to distinguish different instruments in the same category (e.g. an oboe and a clarinet, both woodwind instruments).

The physical characteristics of sound that determine the perception of timbre include spectrum and envelope. Singers and instrumental musicians can change the timbre of the music they are singing/playing by using different singing or playing techniques. For example, a violinist can use different bowing styles or play on different parts of the string to obtain different timbres (e.g., playing sul tasto produces a light, airy timbre, whereas playing sul ponticello produces a harsh, even and aggressive tone). On electric guitar and electric piano, performers can change the timbre using effects units and graphic equalizers.

In simple terms, timbre is what makes a particular musical sound have a different sound from another, even when they have the same pitch and loudness. For instance, it is the difference in sound between a guitar and a piano playing the same note at the same volume. Both instruments can sound equally tuned in relation to each other as they play the same note, and while playing at the same amplitude level each instrument will still sound distinctively with its own unique tone color. Experienced musicians are able to distinguish between different instruments of the same type based on their varied timbres, even if those instruments are playing notes at the same pitch and loudness.

Viral envelope

Some viruses (e.g. HIV and many animal viruses) have viral envelopes covering their protective protein capsids. The envelopes are typically derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins. They may help viruses avoid the host immune system. Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane. The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.

The cell from which the virus itself buds will often die or be weakened and shed more viral particles for an extended period. The lipid bilayer envelope of these viruses is relatively sensitive to desiccation, heat, and detergents, therefore these viruses are easier to sterilize than non-enveloped viruses, have limited survival outside host environments, and typically must transfer directly from host to host. Enveloped viruses possess great adaptability and can change in a short time in order to evade the immune system. Enveloped viruses can cause persistent infections.

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