The A-note to which most musicians tune their instruments oscillates at 440 hertz. As this note has varied by nearly six semitones over time and in different places, the French in 1859 enacted a law to set the pitch at 435 hz. Well, that'll do it. Not right, but it will do it. I mention this not because of any musical ability or knowledge, but simply because today is the anniversary of the promulgation of the French law.
An international treaty in 1939 made A-440 the standard.