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What Is Concert Pitch? 440 Hz vs 432 Hz
One other tuning-related problem took even longer to solve: what to do about a reference frequency. One note and its associated frequency needs to serve as a standard to derive the frequencies for all the other notes, using equal temperament.
After centuries of hair-pulling and fang-gnashing, including weird conspiracy claims about the benefits of 432 Hz vs the evils of 440 Hz, everybody agreed in 1939 that the note A above Middle C would always be tuned to exactly 440 Hz, called Concert pitch or Concert A,and would therefore serve as the reference pitch for setting all the other pitches. (Then World War II started.)