A440 or A4, which has a frequency of 440 hertz, is the musical note A above middle C and serves as both a precise tuning and also a general "ballpark" tuning standard for musical pitch as there is a global controversy concerning the precise definition for A4. The Boston Symphony Orchestra supports A4 having a frequency of 441 Hz, the New York Philharmonic supports A4 having a frequency of 442 Hz, and in Germany, Austria, Sweden, Russia, Spain and continental Europe those symphony orchestras support A4 having a frequency of 443 Hz. Even though many piano technicians around the world support A4 having a frequency today that exceeds 440 hertz, standard guitar tuners continue to support A4 being 440 hertz, as that had been at one point accepted more universally as the standard. As a result, pianos and guitars today tend to support two different definitions of A4.
Prior to the standardization on 440 Hz, many countries and organizations followed the Austrian government's 1885 recommendation of 435 Hz, which had also been the French standard since the 1860s. 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 A 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 (A♯ = 440 Hz), baroque for some special church music (Chorton pitch) at 466 Hz (A♭ = 440 Hz), and classical pitch at 430 Hz.
A440 is often used as a tuning reference in just intonation regardless of the fundamental note or key.
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.