In acoustics, the recognised pitch, i.e. frequency, of the generated sound waves to which musical insutrments are tuned., so that they may play together. The exact value has varied somewhat during musical history, but it was recently internationally standardised so that A (above middle-C) becomes 440 hertz. Dance bands are normally pitched a few hertz lower. Allowance must be made for the rise in temperature in concert halls, in ways peculiar to different types of instrument.