440

The frequency of the sound A is usually set to 440Hz. This is commonly used all of the world.

The American music industry reached their own informal compromise of 440 Hz in 1926, and used 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 (and was reaffirmed by them in 1975) as ISO 16.