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A concert orchestra tunes to the note A at 440 cycles a second. That number is not a law of nature. It is an agreement, so a piano in one city and a violin in another can share a pitch. Before the agreement, halls tuned higher or lower depending on the century and the room.

A440 is a chosen pitch, so instruments can agree.

pitch · Encyclopaedia Britannica · 1936 · Factapp