The **decimal** numeral system (also called **base-ten** positional numeral system, and occasionally called **denary**) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral system.^{[1]} The way of denoting numbers in the decimal system is often referred to as *decimal notation*.^{[2]}

A *decimal numeral*, or just *decimal*, or casually *decimal number*, refers generally to the notation of a number in the decimal numeral system. Decimals may sometimes be identified for containing a decimal separator (for example the "." in 10.00 or 3.14159). "Decimal" may also refer specifically to the digits after the decimal separator, such as in "3.14 is the approximation of π to *two decimals*".

The numbers that may be represented in the decimal system are the decimal fractions, that is the fractions of the form *a*/10^{n}, where *a* is an integer, and *n* is a non-negative integer.

The decimal system has been extended to *infinite decimals*, for representing any real number, by using an infinite sequence of digits after the decimal separator (see Decimal representation). In this context, the decimal numerals with a finite number of non–zero places after the decimal separator are sometimes called *terminating decimals*. A repeating decimal, is an infinite decimal, that, after some place repeats indefinitely the same sequence of digits (for example 5.123144144144144... = 5.123144).^{[3]} An infinite decimal represents a rational number if and only if it is a repeating decimal or has a finite number of nonzero digits.

Many numeral systems of ancient civilisations use ten and its powers for representing numbers, probably because there are ten fingers on two hands and people started counting by using their fingers. Examples are Brahmi numerals, Greek numerals, Hebrew numerals, Roman numerals, and Chinese numerals. Very large numbers were difficult to represent in these old numeral systems, and, only the best mathematicians were able to multiply or divide large numbers. These difficulties were completely solved with the introduction of the Hindu–Arabic numeral system for representing integers. This system has been extended to represent some non-integer numbers, called *decimal fractions* or *decimal numbers* for forming the *decimal numeral system*.

For writing numbers, the decimal system uses ten decimal digits, a decimal mark, and, for negative numbers, a minus sign "−". The decimal digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9;^{[4]} the decimal separator is the dot "." in many countries (including all English speaking ones), but may be a comma "," in other countries (mainly in continental Europe).

For representing a non-negative number, a decimal consists of

- either a (finite) sequence of digits such as 2017, or in full generality,

- (in this case, the (entire) decimal represents an integer)

- or two sequence of digits separated by a decimal mark such as 3.14159, 15.00, or in full generality

It is generally assumed that, if *m* > 0, the first digit *a*_{m} is not zero, but, in some circumstances, it may be useful to have one or more 0's on the left. This does not change the value represented by the decimal. For example, 3.14 = 03.14 = 003.14. Similarly, if *b*_{n} =0, it may be removed, and conversely, trailing zeros may be added without changing the represented number. For example, 15 = 15.0 = 15.00 and 5.2 = 5.20 = 5.200. sometimes, the unnecessary zeros are used for indicating the accuracy of a measurement. For example, 15.00m may indicate that the measurement error is less than one centimeter, while 15m may mean that the length is roughly fifteen meters, and that the error may exceed 10 cm.

For representing a negative number, a minus sign is placed before *a*_{m}.

The numeral represents the number

Therefore, the contribution of each digit to the value of a number depends on its position in the numeral. That is, the decimal system is a positional numeral system

The numbers that are represented by decimal numeral are the **decimal fractions** (sometimes called **decimal numbers**), that is, the rational numbers that may be expressed as a fraction, the denominator of which is a power of ten.^{[5]} For example, the numerals represent the fractions 8/10, 1489/100, 24/100000. More generally, a decimal with *n* digits after the separator represents the fraction with denominator 10^{n}, whose numerator is the integer obtained by removing the separator.

Expressed as a fully reduced fraction, the decimal numbers are those, whose denominator is a product of a power of 2 by a power of 5. Thus the smallest denominators of decimal numbers are

The *integer part*, or *integral part* of a decimal is the integer written to the left of the decimal separator (see also truncation). For a non-negative decimal, it is the largest integer that is not greater than the decimal. The part from the decimal separator to the right is the *fractional part*, which equals the difference between the numeral and its integer part.

When the integral part of a numeral is zero, it may occur, typically in computing, that the integer part is not written (for example .1234, instead of 0.1234). In normal writing, this is generally avoided because of the risk of confusion between the decimal mark and other punctuation.

Decimal numerals do not allow an exact representation for all real numbers, so e.g. for the real number π. Nevertheless, they allow approximating every real number with any desired accuracy, e.g., the decimal 3.14159 approximates the real π, being less than 10^{−5} off; and so decimals are widely used in science, engineering and everyday life.

More precisely, for every real number *x*, and every positive integer *n*, there are two decimals *l* and *u*, with at most *n* digits after the decimal mark, such that *l* ≤ *x* ≤ *u* and (*u* – *l*) = 10^{−n}.

Numbers are very often obtained as the result of a measurement. As measurements are generally afflicted with some measurement error with a known upper bound, the result of a measurement is well represented by a decimal with *n* digits after the decimal mark, as soon as the absolute measurement error is bounded from above by 10^{−n}. In practice, measurement results are often given with a certain number of digits after the decimal point, which indicate the error bounds. For example, although 0.080 and 0.08 denote the same decimal number, the numeral 0.080 is to suggest a measurement with an error less than 0.001, while the numeral 0.08 indicates an absolute error bounded by 0.01. In both cases, the true value of the measured quantity could be, for example, 0.0803 or 0.0796 (see also significant figures).

For a real number *x* and an integer *n* ≥ 0, let [*x*]_{n} denote the (finite) decimal expansion of the greatest number that is not greater than *x*, which has exactly *n* digits after the decimal mark. Let *d*_{i} denote the last digit of [*x*]_{i}. It is straightforward to see that [*x*]_{n} may be obtained by appending *d*_{n} to the right of [*x*]_{n–1}. This way one has

- [
*x*]_{n}= [*x*]_{0}.*d*_{1}*d*_{2}...*d*_{n−1}*d*_{n},

and the difference of [*x*]_{n–1} and [*x*]_{n} amounts to

- |[
*x*]_{n}− [*x*]_{n–1}| =*d*_{n}⋅ 10^{−n}< 10^{−n+1},

which is either 0, if *d*_{n} = 0, or gets arbitrarily small, when *n* tends to infinity. According to the definition of a limit, *x* is the limit of [*x*]_{n} when *n* tends to infinity. This is written as or

*x*= [*x*]_{0}.*d*_{1}*d*_{2}...*d*_{n}...,

which is called an **infinite decimal expansion** of *x*.

Conversely, for any integer [*x*]_{0} and any sequence of digits the (infinite) expression [*x*]_{0}.*d*_{1}*d*_{2}...*d*_{n}... is an *infinite decimal expansion* of a real number *x*. This expansion is unique if neither all *d*_{n} are equal to 9 nor all *d*_{n} are equal to 0 for *n* large enough (for all *n* greater than some natural number *N*).

If all *d*_{n} for *n* > *N* equal to 9 and [*x*]_{n} = [*x*]_{0}.*d*_{1}*d*_{2}...*d*_{n}, the limit of the sequence is the decimal fraction obtained by replacing the last digit that is not a 9, i.e.: *d*_{N}, by *d*_{N} + 1, and replacing all subsequent 9s by 0s (see 0.999...).

Any such decimal fraction, i.e., *d*_{n} = 0 for *n* > *N*, may be converted to its equivalent infinite decimal expansion by replacing *d*_{N} by *d*_{N} − 1, and replacing all subsequent 0s by 9s (see 0.999...).

In summary, every real number that is not a decimal fraction has a unique infinite decimal expansion. Each decimal fraction has exactly two infinite decimal expansions, one containing only 0s after some place, which is obtained by the above definition of [*x*]_{n}, and the other containing only 9s after some place, which is obtained by defining [*x*]_{n} as the greatest number that is *less* than *x*, having exactly *n* digits after the decimal mark.

The long division allows computing the infinite decimal expansion of a rational number. If the rational number is a decimal fraction, the division stops eventually, producing a decimal numeral, which may be prolongated into an infinite expansion by adding infinitely many 0. If the rational number is not a decimal fraction, the division may continue indefinitely. However, as all successive remainder are less than the divisor, there are only a finite number of possible remainders, and after some place, the same sequence of digits must be repeated indefinitely in the quotient. That is, one has a *repeating decimal*. For example,

- 1/81 = 0. 012345679 012... (with the group 012345679 indefinitely repeating).

Conversely, every eventually repeating sequence of digit is the infinite decimal expansion of a rational number. This is a consequence of the fact that the recurring part of a decimal representation is, in fact, an infinite geometric series which will sum to a rational number. For example,

Most modern computer hardware and software systems commonly use a binary representation internally (although many early computers, such as the ENIAC or the IBM 650, used decimal representation internally).^{[6]}
For external use by computer specialists, this binary representation is sometimes presented in the related octal or hexadecimal systems.

For most purposes, however, binary values are converted to or from the equivalent decimal values for presentation to or input from humans; computer programs express literals in decimal by default. (123.1, for example, is written as such in a computer program, even though many computer languages are unable to encode that number precisely.)

Both computer hardware and software also use internal representations which are effectively decimal for storing decimal values and doing arithmetic. Often this arithmetic is done on data which are encoded using some variant of binary-coded decimal,^{[7]}^{[8]} especially in database implementations, but there are other decimal representations in use (such as in the new IEEE 754 Standard for Floating-Point Arithmetic).^{[9]}

Decimal arithmetic is used in computers so that decimal fractional results of adding (or subtracting) values with a fixed length of their fractional part always are computed to this same length of precision. This is especially important for financial calculations, e.g., requiring in their results integer multiples of the smallest currency unit for book keeping purposes. This is not possible in binary, because the negative powers of have no finite binary fractional representation; and is generally impossible for multiplication (or division).^{[10]}^{[11]} See Arbitrary-precision arithmetic for exact calculations.

Many ancient cultures calculated with numerals based on ten, sometimes argued due to human hands typically having ten digits.^{[12]} Standardized weights used in Indus Valley Civilization (c.3300-1300 BCE) were based on the ratios: 1/20, 1/10, 1/5, 1/2, 1, 2, 5, 10, 20, 50, 100, 200, and 500, while their standardized ruler – the *Mohenjo-daro ruler* – was divided into ten equal parts.^{[13]}^{[14]}^{[15]} Egyptian hieroglyphs, in evidence since around 3000 BC, used a purely decimal system,^{[16]} just as the Cretan hieroglyphs (ca. 1625−1500 BC) of the Minoans whose numerals are closely based on the Egyptian model.^{[17]}^{[18]} The decimal system was handed down to the consecutive Bronze Age cultures of Greece, including Linear A (ca. 18th century BC−1450 BC) and Linear B (ca. 1375−1200 BC) — the number system of classical Greece also used powers of ten, including, like the Roman numerals did, an intermediate base of 5.^{[19]} Notably, the polymath Archimedes (ca. 287–212 BC) invented a decimal positional system in his Sand Reckoner which was based on 10^{8}^{[19]} and later led the German mathematician Carl Friedrich Gauss to lament what heights science would have already reached in his days if Archimedes had fully realized the potential of his ingenious discovery.^{[20]} The Hittites hieroglyphs (since 15th century BC), just like the Egyptian and early numerals in Greece, was strictly decimal.^{[21]}

Some non-mathematical ancient texts like the Vedas dating back to 1900–1700 BCE make use of decimals and mathematical decimal fractions.^{[22]}

The Egyptian hieratic numerals, the Greek alphabet numerals, the Hebrew alphabet numerals, the Roman numerals, the Chinese numerals and early Indian Brahmi numerals are all non-positional decimal systems, and required large numbers of symbols. For instance, Egyptian numerals used different symbols for 10, 20, to 90, 100, 200, to 900, 1000, 2000, 3000, 4000, to 10,000.^{[23]}
The world's earliest positional decimal system was the Chinese rod calculus.^{[24]}

Decimal fractions were first developed and used by the Chinese in the end of 4th century BC,^{[25]} and then spread to the Middle East and from there to Europe.^{[24]}^{[26]} The written Chinese decimal fractions were non-positional.^{[26]} However, counting rod fractions were positional.^{[24]}

Qin Jiushao in his book Mathematical Treatise in Nine Sections (1247^{[27]}) denoted 0.96644 by

- 寸
- , meaning
- 寸
- 096644

J. Lennart Berggren notes that positional decimal fractions appear for the first time in a book by the Arab mathematician Abu'l-Hasan al-Uqlidisi written in the 10th century.^{[28]} The Jewish mathematician Immanuel Bonfils used decimal fractions around 1350, anticipating Simon Stevin, but did not develop any notation to represent them.^{[29]} The Persian mathematician Jamshīd al-Kāshī claimed to have discovered decimal fractions himself in the 15th century.^{[28]} Al Khwarizmi introduced fraction to Islamic countries in the early 9th century; a Chinese author has alleged that his fraction presentation was an exact copy of traditional Chinese mathematical fraction from Sunzi Suanjing.^{[24]} This form of fraction with numerator on top and denominator at bottom without a horizontal bar was also used by 10th century Abu'l-Hasan al-Uqlidisi and 15th century Jamshīd al-Kāshī's work "Arithmetic Key".^{[24]}^{[30]}

A forerunner of modern European decimal notation was introduced by Simon Stevin in the 16th century.^{[31]}

The ingenious method of expressing every possible number using a set of ten symbols emerged in India. Several Indian languages show a straightforward decimal system. Many Indo-Aryan and Dravidian languages have numbers between 10 and 20 expressed in a regular pattern of addition to 10.^{[32]}

The Hungarian language also uses a straightforward decimal system. All numbers between 10 and 20 are formed regularly (e.g. 11 is expressed as "tizenegy" literally "one on ten"), as with those between 20 and 100 (23 as "huszonhárom" = "three on twenty").

A straightforward decimal rank system with a word for each order (10 十, 100 百, 1000 千, 10,000 万), and in which 11 is expressed as *ten-one* and 23 as *two-ten-three*, and 89,345 is expressed as 8 (ten thousands) 万 9 (thousand) 千 3 (hundred) 百 4 (tens) 十 5 is found in Chinese, and in Vietnamese with a few irregularities. Japanese, Korean, and Thai have imported the Chinese decimal system. Many other languages with a decimal system have special words for the numbers between 10 and 20, and decades. For example, in English 11 is "eleven" not "ten-one" or "one-teen".

Incan languages such as Quechua and Aymara have an almost straightforward decimal system, in which 11 is expressed as *ten with one* and 23 as *two-ten with three*.

Some psychologists suggest irregularities of the English names of numerals may hinder children's counting ability.^{[33]}

Some cultures do, or did, use other bases of numbers.

- Pre-Columbian Mesoamerican cultures such as the Maya used a base-20 system (perhaps based on using all twenty fingers and toes).
- The Yuki language in California and the Pamean languages
^{[34]}in Mexico have octal (base-8) systems because the speakers count using the spaces between their fingers rather than the fingers themselves.^{[35]} - The existence of a non-decimal base in the earliest traces of the Germanic languages, is attested by the presence of words and glosses meaning that the count is in decimal (cognates to ten-count or tenty-wise), such would be expected if normal counting is not decimal, and unusual if it were.
^{[36]}^{[37]}Where this counting system is known, it is based on the long hundred of 120 in number, and a long thousand of 1200 in number. The descriptions like 'long' only appear after the small hundred of 100 in number appeared with the Christians. Gordon's Introduction to Old Norse p 293, gives number names that belong to this system. An expression cognate to 'one hundred and eighty' is translated to 200, and the cognate to 'two hundred' is translated at 240. Goodare details the use of the long hundred in Scotland in the Middle Ages, giving examples, calculations where the carry implies i C (i.e. one hundred) as 120, etc. That the general population were not alarmed to encounter such numbers suggests common enough use. It is also possible to avoid hundred-like numbers by using intermediate units, such as stones and pounds, rather than a long count of pounds. Goodare gives examples of numbers like vii score, where one avoids the hundred by using extended scores. There is also a paper by W.H. Stevenson, on 'Long Hundred and its uses in England'. - Many or all of the Chumashan languages originally used a base-4 counting system, in which the names for numbers were structured according to multiples of 4 and 16.
^{[38]} - Many languages
^{[39]}use quinary (base-5) number systems, including Gumatj, Nunggubuyu,^{[40]}Kuurn Kopan Noot^{[41]}and Saraveca. Of these, Gumatj is the only true 5–25 language known, in which 25 is the higher group of 5. - Some Nigerians use duodecimal systems.
^{[42]}So did some small communities in India and Nepal, as indicated by their languages.^{[43]} - The Huli language of Papua New Guinea is reported to have base-15 numbers.
^{[44]}*Ngui*means 15,*ngui ki*means 15 × 2 = 30, and*ngui ngui*means 15 × 15 = 225. - Umbu-Ungu, also known as Kakoli, is reported to have base-24 numbers.
^{[45]}*Tokapu*means 24,*tokapu talu*means 24 × 2 = 48, and*tokapu tokapu*means 24 × 24 = 576. - Ngiti is reported to have a base-32 number system with base-4 cycles.
^{[39]} - The Ndom language of Papua New Guinea is reported to have base-6 numerals.
^{[46]}*Mer*means 6,*mer an thef*means 6 × 2 = 12,*nif*means 36, and*nif thef*means 36×2 = 72.

- Algorism
- Binary-coded decimal (BCD)
- Decimal computer
- Decimal representation
- Decimal separator
- Densely packed decimal (DPD)
- Dewey Decimal Classification (DDC)
- Duodecimal
- Octal
- Scientific notation
- SI prefix

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^{a}^{b}Greek numerals **^**Menninger, Karl:*Zahlwort und Ziffer. Eine Kulturgeschichte der Zahl*, Vandenhoeck und Ruprecht, 3rd. ed., 1979, ISBN 3-525-40725-4, pp. 150–153**^**Georges Ifrah:*From One to Zero. A Universal History of Numbers*, Penguin Books, 1988, ISBN 0-14-009919-0, pp. 218f. (The Hittite hieroglyphic system)**^**(Atharva Veda 5.15, 1-11)**^**Lam Lay Yong et al. The Fleeting Footsteps pp 137–139- ^
^{a}^{b}^{c}^{d}^{e}Lam Lay Yong, "The Development of Hindu–Arabic and Traditional Chinese Arithmetic",*Chinese Science*, 1996 p38, Kurt Vogel notation **^**"Ancient bamboo slips for calculation enter world records boo".*The Institute of Archaeology, Chinese Academy of Social Sciences*. Retrieved 10 May 2017.- ^
^{a}^{b}Joseph Needham (1959). "Decimal System".*Science and Civilisation in China, Volume III, Mathematics and the Sciences of the Heavens and the Earth*. Cambridge University Press. **^**Jean-Claude Martzloff, A History of Chinese Mathematics, Springer 1997 ISBN 3-540-33782-2- ^
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Alpha (uppercase Α, lowercase α; Ancient Greek: ἄλφα, álpha, modern pronunciation álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of 1.

It was derived from the Phoenician and Hebrew letter aleph - an ox or leader.Letters that arose from alpha include the Latin A and the Cyrillic letter А.

In English, the noun "alpha" is used as a synonym for "beginning", or "first" (in a series), reflecting its Greek roots.

Binary-coded decimalIn computing and electronic systems, binary-coded decimal (BCD) is a class of binary encodings of decimal numbers where each decimal digit is represented by a fixed number of bits, usually four or eight. Special bit patterns are sometimes used for a sign or for other indications (e.g., error or overflow).

In byte-oriented systems (i.e. most modern computers), the term unpacked BCD usually implies a full byte for each digit (often including a sign), whereas packed BCD typically encodes two decimal digits within a single byte by taking advantage of the fact that four bits are enough to represent the range 0 to 9. The precise 4-bit encoding may vary however, for technical reasons, see Excess-3 for instance. The ten states representing a BCD decimal digit are sometimes called tetrades (for the nibble typically needed to hold them also known as tetrade) with those don't care-states unused named pseudo-tetrad(e)s or pseudo-decimal digit).BCD's main virtue is its more accurate representation and rounding of decimal quantities as well as an ease of conversion into human-readable representations, in comparison to binary positional systems. BCD's principal drawbacks are a small increase in the complexity of the circuits needed to implement basic arithmetics and a slightly less dense storage.

BCD was used in many early decimal computers, and is implemented in the instruction set of machines such as the IBM System/360 series and its descendants, Digital Equipment Corporation's VAX, the Burroughs B1700, and the Motorola 68000-series processors. Although BCD per se is not as widely used as in the past and is no longer implemented in newer computers' instruction sets (such as ARM; x86 does not support its BCD instructions in long mode any more), decimal fixed-point and floating-point formats are still important and continue to be used in financial, commercial, and industrial computing, where subtle conversion and fractional rounding errors that are inherent in floating point binary representations cannot be tolerated.

Binary numberIn mathematics and digital electronics, a binary number is a number expressed in the base-2 numeral system or binary numeral system, which uses only two symbols: typically "0" (zero) and "1" (one).

The base-2 numeral system is a positional notation with a radix of 2. Each digit is referred to as a bit. Because of its straightforward implementation in digital electronic circuitry using logic gates, the binary system is used by almost all modern computers and computer-based devices.

Decimal degreesDecimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.

Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime Meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.

Decimal separatorA decimal separator is a symbol used to separate the integer part from the fractional part of a number written in decimal form.

Different countries officially designate different symbols for the decimal separator. The choice of symbol for the decimal separator also affects the choice of symbol for the thousands separator used in digit grouping, so the latter is also treated in this article.

Any such symbol can be called a decimal mark, decimal marker or decimal sign. But symbol-specific names are also used; decimal point and decimal comma refer to an (either baseline or middle) dot and comma respectively, when it is used as a decimal separator; these are the usual terms used in English, with the aforementioned generic terms reserved for abstract usage.In many contexts, when a number is spoken, the function of the separator is assumed by the spoken name of the symbol: comma or point in most cases. In some specialized contexts, the word decimal is instead used for this purpose (such as in ICAO-regulated air traffic control communications).

In mathematics the decimal separator is a type of radix point, a term that also applies to number systems with bases other than ten.

Delta (letter)Delta (uppercase Δ, lowercase δ or 𝛿; Greek: δέλτα délta, [ˈðelta]) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter dalet 𐤃, Letters that come from delta include Latin D and Cyrillic Д.

A river delta (originally, the Nile River delta) is so named because its shape approximates the triangular uppercase letter delta. Despite a popular legend, this use of the word delta was not coined by Herodotus.

Dewey Decimal ClassificationThe Dewey Decimal Classification (DDC), colloquially the Dewey Decimal System, is a proprietary library classification system first published in the United States by Melvil Dewey in 1876. Originally described in a four-page pamphlet, it has been expanded to multiple volumes and revised through 23 major editions, the latest printed in 2011. It is also available in an abridged version suitable for smaller libraries. OCLC, a non-profit cooperative that serves libraries, currently maintains the system and licenses online access to WebDewey, a continuously updated version for catalogers.

The Decimal Classification introduced the concepts of relative location and relative index which allow new books to be added to a library in their appropriate location based on subject. Libraries previously had given books permanent shelf locations that were related to the order of acquisition rather than topic. The classification's notation makes use of three-digit Arabic numerals for main classes, with fractional decimals allowing expansion for further detail. Using Arabic numerals for symbols, it is flexible to the degree that numbers can be expanded in linear fashion to cover special aspects of general subjects. A library assigns a classification number that unambiguously locates a particular volume in a position relative to other books in the library, on the basis of its subject. The number makes it possible to find any book and to return it to its proper place on the library shelves. The classification system is used in 200,000 libraries in at least 135 countries.

DuodecimalThe duodecimal system (also known as base 12 or dozenal) is a positional notation numeral system using twelve as its base. The number twelve (that is, the number written as "12" in the base ten numerical system) is instead written as "10" in duodecimal (meaning "1 dozen and 0 units", instead of "1 ten and 0 units"), whereas the digit string "12" means "1 dozen and 2 units" (i.e. the same number that in decimal is written as "14"). Similarly, in duodecimal "100" means "1 gross", "1000" means "1 great gross", and "0.1" means "1 twelfth" (instead of their decimal meanings "1 hundred", "1 thousand", and "1 tenth").

The number twelve, a superior highly composite number, is the smallest number with four non-trivial factors (2, 3, 4, 6), and the smallest to include as factors all four numbers (1 to 4) within the subitizing range, and the smallest abundant number. As a result of this increased factorability of the radix and its divisibility by a wide range of the most elemental numbers (whereas ten has only two non-trivial factors: 2 and 5, and not 3, 4, or 6), duodecimal representations fit more easily than decimal ones into many common patterns, as evidenced by the higher regularity observable in the duodecimal multiplication table. As a result, duodecimal has been described as the optimal number system. Of its factors, 2 and 3 are prime, which means the reciprocals of all 3-smooth numbers (such as 2, 3, 4, 6, 8, 9, 12, 16, 18, 24, 27, 32, 36, ...) have a terminating representation in duodecimal. In particular, the five most elementary fractions ( 1⁄2, 1⁄3, 2⁄3, 1⁄4 and 3⁄4) all have a short terminating representation in duodecimal (0.6, 0.4, 0.8, 0.3 and 0.9, respectively), and twelve is the smallest radix with this feature (because it is the least common multiple of 3 and 4). This all makes it a more convenient number system for computing fractions than most other number systems in common use, such as the decimal, vigesimal, binary, octal and hexadecimal systems. Although the trigesimal and sexagesimal systems (where the reciprocals of all 5-smooth numbers terminate) do even better in this respect, this is at the cost of unwieldy multiplication tables and a much larger number of symbols to memorize.

EBCDICExtended Binary Coded Decimal Interchange Code (EBCDIC; ) is an eight-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems. It descended from the code used with punched cards and the corresponding six bit binary-coded decimal code used with most of IBM's computer peripherals of the late 1950s and early 1960s. It is supported by various non-IBM platforms, such as Fujitsu-Siemens' BS2000/OSD, OS-IV, MSP, and MSP-EX, the SDS Sigma series, Unisys VS/9, Burroughs MCP and ICL VME.

Fraction (mathematics)A **fraction** (from Latin * fractus*, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, three-quarters. A

We begin with positive common fractions, where the numerator and denominator are natural numbers. The numerator represents a number of equal parts, and the denominator indicates how many of those parts make up a unit or a whole. The denominator cannot be zero because zero parts can never make up a whole. For example, in the fraction 3/4, the numerator, 3, tells us that the fraction represents 3 equal parts, and the denominator, 4, tells us that 4 parts make up a whole. The picture to the right illustrates or ^{3}⁄_{4} of a cake.

A common fraction is a numeral which represents a rational number. That same number can also be represented as a decimal, a percent, or with a negative exponent. For example, 0.01, 1%, and 10^{−2} all equal the fraction 1/100. An integer such as the number 7 can be thought of as having an implicit denominator of one: 7 equals 7/1.

Other uses for fractions are to represent ratios and division. Thus the fraction 3/4 is also used to represent the ratio 3:4 (the ratio of the part to the whole) and the division 3 ÷ 4 (three divided by four). The non-zero denominator in the case using a fraction to represent division is an example of the rule that division by zero is undefined.

We can also write negative fractions, which represent the opposite of a positive fraction. For example if 1/2 represents a half dollar profit, then −1/2 represents a half dollar loss. Because of the rules of division of signed numbers, which require that, for example, negative divided by positive is negative, −1/2, -1/2, 1/-2, and −-1/-2 all represent the same fraction, negative one-half. Because negative divided by negative is positive, -1/-2 represents positive one-half.

In mathematics the set of all numbers that can be expressed in the form a/b, where a and b are integers and b is not zero, is called the set of rational numbers and is represented by the symbol **Q**, which stands for quotient. The test for a number being a rational number is that it can be written in that form (i.e., as a common fraction). However, the word *fraction* is also used to describe mathematical expressions that are not rational numbers, for example algebraic fractions (quotients of algebraic expressions), and expressions that contain irrational numbers, such as √2/2 (see square root of 2) and π/4 (see proof that π is irrational).

In mathematics and computing, hexadecimal (also base 16, or hex) is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols "0"–"9" to represent values zero to nine, and "A"–"F" (or alternatively "a"–"f") to represent values ten to fifteen.

Hexadecimal numerals are widely used by computer system designers and programmers, as they provide a more human-friendly representation of binary-coded values. Each hexadecimal digit represents four binary digits, also known as a nibble, which is half a byte. For example, a single byte can have values ranging from 0000 0000 to 1111 1111 in binary form, which can be more conveniently represented as 00 to FF in hexadecimal.

In mathematics, a subscript is typically used to specify the radix. For example the decimal value 10,995 would be expressed in hexadecimal as 2AF316. In programming, a number of notations are used to support hexadecimal representation, usually involving a prefix or suffix. The prefix 0x is used in C and related languages, which would denote this value by 0x2AF3.

Hexadecimal is used in the transfer encoding Base16, in which each byte of the plaintext is broken into two 4-bit values and represented by two hexadecimal digits.

InchThe inch (abbreviation: in or ″) is a unit of length in the (British) imperial and United States customary systems of measurement. It is equal to 1⁄36 yard or 1⁄12 of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb. Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s it has been based on the metric system and defined as exactly 2.54 cm.

List of Dewey Decimal classesThe Dewey Decimal Classification (DDC) is structured around ten main classes covering the entire world of knowledge; each main class is further structured into ten hierarchical divisions, each having ten sections of increasing specificity. As a system of library classification the DDC is "arranged by discipline, not subject", so a topic like clothing is classed based on its disciplinary treatment (psychological influence of clothing at 155.95, customs associated with clothing at 391, and fashion design of clothing at 746.92) within the conceptual framework. The list below presents the ten main classes, hundred divisions, and thousand sections.

Metric prefixA metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well. Each prefix has a unique symbol that is prepended to the unit symbol. The prefix kilo-, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli-, likewise, may be added to metre to indicate division by one thousand; one millimetre is equal to one thousandth of a metre.

Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six dating back to the system's introduction in the 1790s. Metric prefixes have even been prepended to non-metric units. The SI prefixes are standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991. Since 2009, they have formed part of the International System of Quantities.

OmegaOmega (capital: Ω, lowercase: ω; Greek ὦ, later ὦ μέγα, Modern Greek ωμέγα) is the 24th and last letter of the Greek alphabet. In the Greek numeric system/Isopsephy (Gematria), it has a value of 800. The word literally means "great O" (ō mega, mega meaning "great"), as opposed to omicron, which means "little O" (o mikron, micron meaning "little").In phonetic terms, the Ancient Greek Ω is a long open-mid o [ɔː], comparable to the vowel of British English raw. In Modern Greek, Ω represents the mid back rounded vowel /o/, the same sound as omicron. The letter omega is transcribed ō or simply o.

As the last letter of the Greek alphabet, Omega is often used to denote the last, the end, or the ultimate limit of a set, in contrast to alpha, the first letter of the Greek alphabet.

RadixIn mathematical numeral systems, the radix or base is the number of unique digits, including the digit zero, used to represent numbers in a positional numeral system. For example, for the decimal system (the most common system in use today) the radix is ten, because it uses the ten digits from 0 through 9.

In any standard positional numeral system, a number is conventionally written as (x)y with x as the string of digits and y as its base, although for base ten the subscript is usually assumed (and omitted, together with the pair of parentheses), as it is the most common way to express value. For example, (100)dec = 100 (in the decimal system) represents the number one hundred, while (100)2 (in the binary system with base 2) represents the number four.

Scientific notationScientific notation (also referred to as scientific form or standard index form, or standard form in the UK) is a way of expressing numbers that are too big or too small to be conveniently written in decimal form. It is commonly used by scientists, mathematicians and engineers, in part because it can simplify certain arithmetic operations. On scientific calculators it is usually known as "SCI" display mode.

In scientific notation all numbers are written in the form

m × 10n(m times ten raised to the power of n), where the exponent n is an integer, and the coefficient m is any real number. The integer n is called the

order of magnitude and the real number m is called the significand or mantissa. However, the term "mantissa" may cause confusion because it is the name of the fractional part of the common logarithm. If the number is negative then a minus sign precedes m (as in ordinary decimal notation). In normalized notation, the exponent is chosen so that the absolute value of the coefficient is at least one but less than ten.

Decimal floating point is a computer arithmetic system closely related to scientific notation.

Significant figuresThe significant figures (also known as the significant digits) of a number are digits that carry meaning contributing to its measurement resolution. This includes all digits except:

All leading zeros;

Trailing zeros when they are merely placeholders to indicate the scale of the number (exact rules are explained at identifying significant figures); and

Spurious digits introduced, for example, by calculations carried out to greater precision than that of the original data, or measurements reported to a greater precision than the equipment supports.Significance arithmetic are approximate rules for roughly maintaining significance throughout a computation. The more sophisticated scientific rules are known as propagation of uncertainty.

Numbers are often rounded to avoid reporting insignificant figures. For example, it would create false precision to express a measurement as 12.34500 kg (which has seven significant figures) if the scales only measured to the nearest gram and gave a reading of 12.345 kg (which has five significant figures). Numbers can also be rounded merely for simplicity rather than to indicate a given precision of measurement, for example, to make them faster to pronounce in news broadcasts.

Swatch Internet TimeSwatch Internet Time (or .beat time) is a decimal time concept introduced in 1998 by the Swatch corporation as part of their marketing campaign for their line of "Beat" watches.

Instead of hours and minutes, the mean solar day is divided into 1000 parts called ".beats". Each .beat is equal to one decimal minute in the French Revolutionary decimal time system and lasts 1 minute and 26.4 seconds (86.4 seconds) in standard time. Times are notated as a 3-digit number out of 1000 after midnight. So, @248 would indicate a time 248 .beats after midnight representing 248/1000 of a day, just over 5 hours and 57 minutes.

There are no time zones in Swatch Internet Time; instead, the new time scale of Biel Meantime (BMT) is used, based on Swatch's headquarters in Biel, Switzerland and equivalent to Central European Time, West Africa Time, and UTC+01. Unlike civil time in Switzerland and many other countries, Swatch Internet Time does not observe daylight saving time.

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