When was standard pitch in music agreed?
The US music industry suggested A440 Hz as the standard pitch in
1925 and it was adopted throughout the US in 1926. In 1955, the
International Organization for Standardization adopted A440 Hz as
the world tuning standard.
However, it had been suggested and used much earlier than that
in isolated areas. The Paris Conservatoire adopted A440 as the
"modern concert pitch" as early as 1812.