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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. Johann Heinrich Scheibler recommended A440 as a standard in 1834 after inventing the “tonometer” to measure pitch, and it was approved by the Society of German Natural Scientists and Physicians the same year.
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. This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16.