Orchestras didn't always agree what an 'A' sounds like.
Today most music is tuned so the A above middle C vibrates at 440 times a second. For centuries, pitch varied from city to city, even organ to organ. An international conference in London in 1939 recommended 440, and a standards body made it official in 1975.
When an orchestra tunes up before a concert, the oboe usually plays an A, and everyone matches it. That A is normally set at 440 hertz: the air vibrates 440 times per second. This shared reference, called concert pitch, lets instruments made in different countries play together.
It has not always been so. In the Baroque era, pitch varied widely. Surviving organs and tuning forks show that an A could be well below or above today's pitch depending on the place and period; Handel's tuning fork is said to have sounded around 422 hertz. Musicians travelling between cities often faced different standards.
During the 19th century, pitch tended to creep upward, partly because higher tuning made orchestras sound brighter. Singers complained about the strain. In 1859, the French government set a standard of 435 hertz, known as the diapason normal. Other countries adopted various standards, and confusion persisted.
In 1939, an international conference in London recommended 440 hertz for the A above middle C. The International Organization for Standardization confirmed it as ISO 16 in 1975. Even so, not everyone follows it exactly: many orchestras tune slightly higher, to 442 or 443, for brilliance, and early-music ensembles often use lower pitches to match period instruments. Occasional online claims that 432 hertz is 'natural' or healing have no scientific basis.
Source: Wikipedia — A440 (pitch standard) · Text summarised from Wikipedia (CC BY-SA 4.0)
