“In 1953, a worldwide agreement was signed. Signatories declared that middle “A” on the piano be forevermore tuned to exactly 440 Hz. This frequency became the standard ISO-16 reference for tuning all musical instruments based on the chromatic scale, the one most often used for music in the West.
All the other notes are tuned in standard mathematical ratios leading to and from 440 Hz.
This tone standard is now universally accepted, which is why a piano in Toronto sounds exactly the same as a piano in China.
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Gather ’round, kids. Those of you with tinfoil hats may wish to ensure that they’re fitted snuggly. What I’m about to tell you will shake your faith in all the music you’ve heard in your life If you look down the right paths, it becomes clear that governments and various security apparatuses have used music
Source: The great 440 Hz conspiracy, and why all our music is ‘wrong’ | Principia Scientific Intl.