A440 (pitch standard)
A440 or A4 (also known as the Stuttgart pitch), which has a frequency of 440 Hz, is the musical note A above middle C and serves as a general tuning standard for musical pitch.
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 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. Although not universally accepted, since then it has served as the audio frequency reference for the calibration of acoustic equipment and the tuning of pianos, violins, and other musical instruments.
It is designated A4 in scientific pitch notation because it occurs in the octave that starts with the fourth C key on a standard 88-key piano keyboard. On MIDI, it is note 69.
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.
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