Tuning fork on a resonating box. Via Wikimedia Commons.
When the oboe gives the A and the orchestra tunes up, that sound vibrates four hundred and forty times a second. It looks like a fact of nature, like the boiling point of water. It is not: it is the result of a century of international squabbling, and the date of the verdict is 1939.
Through the whole eighteenth century and half the nineteenth, every city had its own A. The historical tuning forks measured by scholars tell of total anarchy: 409 vibrations in Paris in Rameau’s day, 422 for the fork that belonged to Handel, 436 at La Scala in the mid-nineteenth century, over 450 in certain Viennese orchestras. And the trend was upward, because a higher tuning makes the strings sound brighter: makers raised it, theatres followed, singers paid the bill, shrieking notes written for a world half a tone lower.
The first to put a law on it was the French state: in 1859 a commission that included Rossini, Meyerbeer and Berlioz fixed the diapason normal at 435 vibrations, and France made it mandatory by decree. Italy tried to go lower: in 1884 a ministerial commission recommended an A of 432, with the explicit blessing of Verdi, who in a letter asked for a single tuning for the entire musical world. It is from that letter that the modern mythology of 432 as a cosmic frequency descends, deformed: Verdi was not talking about the vibrations of the universe, he was talking about the throats of baritones.
The final word came in May 1939, in London: an international conference promoted by the broadcasting organisations fixed A at 440 hertz. The choice rewarded technical practicality, since 440 was easy to generate with the radio equipment of the day, and in 1955 the ISO enshrined it as the world standard, the ISO 16 standard, still in force. Naturally orchestras have kept up their little games: Vienna and Berlin habitually tune to 443-445, to shine a little brighter than the others, and baroque orchestras have dropped, by philological choice, to 415, the semitone below, reconstructed from period tuning forks.
The moral is worth keeping in mind every time someone invokes naturalness in music: the sound on which every orchestra on the planet tunes was not decided by nature, it was decided by a commission of bureaucrats and a radio broadcaster, in the year the war broke out. It works beautifully. But it is a convention, like tempos measured against the pulse: music is full of laws dressed up as nature, and recognising them is half the pleasure of listening.