For most of musical history there was no such thing as the right pitch. Through the nineteenth century European orchestras kept creeping sharp, because a brighter sound was fashionable and no authority held the line. Concert A, the note everyone tunes to, drifted upward from the low 420s into the 450s, and there was no single agreed concert pitch to stop it. What sounded thrilling for an orchestra was quietly punishing for singers.

As the reference climbed, written notes sat higher than the composers had ever heard them, and voices were pushed toward the tops of their range night after night, straining to reach music that had been comfortable a generation earlier. The drift had human costs, paid in the throats of the people onstage. France was the first to intervene by law. In 1859 it legislated the diapason normal, fixing concert A at 435 Hz, the first time any government wrote a specific pitch into its statutes. It was a remarkable act, a nation deciding that a frequency was a matter of public order, and it gave the rest of Europe a fixed point to argue around.

Agreement across borders took longer. An international conference met in London in 1939 and settled on A equal to 440 Hz, the value most of the world uses today. That figure was later codified as ISO 16 in 1955, turning a conference resolution into a formal international standard. The number that seems so obviously fixed, the anchor beneath every tuned instrument, was in truth hard-won: the end of a long, sharp climb, brought to rest first by French law and finally by international agreement.