International Concert Pitch

'International Concert Pitch' is a standardization of pitch that sets the frequency of a simple tone at pitch A4 (the A above middle C) to be 440 Hz.

In 1939, an international conference recommended that the A above middle C be tuned to 440 Hz, now a standard known as International Concert Pitch. This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16. The difference between this and the diapason normal is due to confusion over which temperature the French standard should be measured at. The initial standard was A = 439 Hz, but this was superseded by A = 440 Hz after complaints that 439 Hz was difficult to reproduce in a laboratory owing to 439 being a prime number.