A standard where the note A above middle C is tuned to 440Hz. The first tuning standard, A at 435Hz, was presented by the Austrian government in 1885. but around 1926, American manufacturers began using A440. The new standard was recommended by the American Standards Association in 1936, then by the International Organization for Standardization in 1955 and 1975.
Though A440 is the widely accepted “standard,” at least for Western music, other reference tuning pitches are used for period instruments and performances, such as 415Hz, 430Hz, and 466Hz.
