unequal temperament

A system of tuning, especially on old keyboard instruments, in which some of the accidentals were treated as sharps according to just intonation and some as flats (for instance, F sharp, not G flat; B flat, not A sharp). An attempt was thus made to make some of the more frequently used keys come nearer to just intonation than is possible in the tempered scale of the modern piano, but the result was also that the more extreme sharp and flat keys were out of tune. This is one of the chief reasons why these keys were rarely used by early composers.