When a group of musicians uses a particular pitch reference for a performance, the pitch they all tune to is called concert pitch. For instance, a rock group could all decide to tune their guitars down a half step in order to make the high notes less of a strain on their voices. For them, concert pitch would be A415, normally the frequency of A flat.
Most instruments today are tuned so that the A above middle C is 440 Hz, but that wasn't always the case. A440 was first proposed in 1824 at the Stuttgart Conference in Germany, but it wasn't until 1935 that an international conference recommended A440, and it wasn't until 1955 that the International Standards Organization (ISO) adopted it.
Historically concert pitch varied widely. In the 1700s the A above middle C could have been as low as 320 Hz, over a fourth lower than today, and A could have also been as high as 480, almost a whole step higher.
In 1859 the French government passed a law requiring concert pitch to be A435, in order to prevent injury to singers. The British still used A452 until 1896, when it was lowered to 439.
Today most bands and orchestras tune to A440, although some orchestras in the US and Europe may make concert pitch slightly higher at either 442 or 443.
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