A440 is the 440 Hz tone that serves as the standard for musical pitch. A440 is the musical note A above middle C (A4).
In 1936, an international conference 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 (and was reaffirmed by them in 1975) as ISO 16. Since then it has served as the audio frequency reference for the calibration of pianos, violins, and other musical instruments.
Before chromatic tuners, I used an A440 tuning fork to tune my guitar.
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