A440 or Concert A or Middle A is the 440 Hz tone that serves as the standard for musical pitch. A440 is the musical note A above middle C (C4). The term literally means that the A above Middle C = 440 vibrations per second.

Prior to the standardization on 440 Hz, many countries and organizations followed the 435 Hz recommendation the Austrian government made in 1885. The American music industry reached their own informal compromise of 440 Hz in 1926, and used it in instrument manufacturing. In 1936, the American Standards Association recommended that the A above middle C be tuned to 440 Hz.[1] This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16 [2]. Although still not completely universally accepted, since then it has served as the audio frequency reference for the calibration of pianos, violins, and other musical instruments.

In the period instrument movement, a consensus has arisen around a modern "baroque pitch" of 415Hz.