The International Organization for Standardization classifies it as ISO 16. Prior to the standardization on 440 Hz, other frequencies were standardized upon. Although not universally accepted, it serves as the audio frequency reference for the calibration of acoustic equipment and the tuning of pianos, violins, and other musical instruments.
Prior to the standardization on 440 Hz, many countries and organizations followed the French standard since the 1860s of 435 Hz, which had also been the Austrian government's 1885 recommendation. Johann Heinrich Scheibler recommended A440 as a standard in 1834 after inventing the "tonometer" to measure pitch, and it was approved by the German Natural History Society the same year.
The American music industry reached an informal standard of 440 Hz in 1926, and some began using it in instrument manufacturing. In 1936 the American Standards Association recommended that the A above middle C be tuned to 440 Hz. This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16.
A440 is widely used as concert pitch in the United Kingdom and the United States. In continental Europe the frequency of A4 commonly varies between 440 Hz and 444 Hz. In the period instrument movement, a consensus has arisen around a modern baroque pitch of 415 Hz (with 440 Hz corresponding to A♯), baroque for some special church music (Chorton pitch) at 466 Hz (with 440 Hz corresponding to A♭), and classical pitch at 430 Hz.
5 seconds of sine wave at 440 Hz
The US time and frequency station WWV broadcasts a 440 Hz signal at two minutes past every hour, with WWVH broadcasting the same tone at the first minute past every hour. This was added in 1936 to aid orchestras in tuning their instruments.