Equal temperament is a system devised in the 16th cent. to make each step (semitone) of a chromatic scale the same. Though scientifically this is impossible, the ear gratefully accommodates it. C♯ and D♭ on the piano sound the same, but to a string or wind player a C♯ is actually higher than a D♭. Though J S Bach's Well-Tempered Klavier celebrates the idea, the system was not fully adopted until the 19th cent. — Amanda Holden