semitone
The smallest interval in Western music, equal to one half step.
In Depth
A semitone (or half step) is the smallest interval in standard Western music — the distance between two adjacent keys on a piano, whether the keys are black-white or white-white. Twelve semitones make up an octave, and the entire chromatic scale consists of consecutive semitones.
The semitone is the fundamental building block of the Western pitch system. Major scales are built from a specific pattern of whole tones and semitones (W-W-H-W-W-W-H), while the pattern differs for minor scales. In equal temperament — the tuning system used on modern pianos and fretted instruments — all semitones are exactly the same size, which is a compromise that allows music in any key to sound acceptable.
The equal temperament system — which divides the octave into 12 equal semitones — was first calculated mathematically by Chinese scholar Zhu Zaiyu in 1584, decades before European theorists worked it out.