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Before standardization on 440 Hz, many countries and organizations followed the French standard since the 1860s of 435 Hz, which had also been the Austrian government’s 1885 recommendation.[2] Johann Heinrich Scheibler recommended A440 as a standard in 1834 after inventing the “tonometer” to measure pitch,[3] and it was approved by the Society of German Natural Scientists and Physicians the same year.[4]
The American music industry reached an informal standard of 440 Hz in 1926, and some began using it in instrument manufacturing.
In 1936, the American Standards Association recommended that the A above middle C be tuned to 440 Hz.[5] This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16.[6]
It is designated A4 in scientific pitch notation because it occurs in the octave that starts with the fourth C key on a standard 88-key piano keyboard. On MIDI, A440 is note 69 (0x45 hexadecimal).