The A 440 Pitch Standard

In 1939, an International Conference met in London and unanimously adopted 440 Hz as the standard frequency for the pitch A4, and that is the almost universal standard at present. The National Institute of Standards and Technology (NIST) broadcasts a precise 440 Hz reference tone on its short wave radio station WWV.

Rossing reports Handel's tuning fork frequency to be 422.5 Hz for that A, and the eras of Hayden, Mozart, Bach and Beethoven had pitch standards around that frequency. This means that their compositions are now played about 70 cents sharper than the originals.