International Concert Pitch

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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.[1]

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in 1955 (and was reaffirmed by them in 1975) as ISO 16.[2] The initial standard was A = 439 Hz, but this was superseded by A = 440 Hz after complaints that 439 Hz was difficult to reproduce in a laboratory owing to 439 being a prime number.[3]

The French standard for A4 at 435 Hz, the diapason normal, should be measured at 15°C, and a change to 20°C causes a shift to very nearly 440 Hz in wind instruments.[4] Of course, tuning forks do not share the same temperature dependence.


  1. William Vennard (1967). “Pitch”, Singing: The Mechanism and the Technic, 5th ed. Carl Fischer, LLC, p. 3. ISBN 0825800552. 
  2. ISO 16:1975; Acoustics -- Standard tuning frequency (Standard musical pitch). ISO Standards. Retrieved on 2012-07-05.
  3. Lynn Cavanagh. A brief history of the establishment of international standard pitch a=440 hertz (PDF). WAM: Webzin o audiju i muzici. Retrieved on 2012-06-27.
  4. George Ashdown Audsley (1965). The Art of Organ-building: A Comprehensive Historical, Theoretical, and Practical Treatise on the Tonal Appointment and Mechanical Construction of Concert-room, Church, and Chamber Organs, Volume 2, Reprint of 1905 ed. Courier Dover Publications, p. 636. ISBN 0486213153.