Semitones and tones

A semitone is the smallest distance between two notes in standard Western tuning. On a keyboard it is the very next key along — black or white, no skipping.

A tone is two semitones.

That is the whole definition. It matters because every scale formula, every chord formula and every interval size is measured in these units, and none of them means anything until this one does.

The names differ by country

They are the same things. This site uses semitone and tone, and mentions the American terms where a formula is usually written with them.

Counting them

The counting mistake almost everyone makes once: skipping the black keys. C to D looks adjacent because the white keys are adjacent, but there is a key between them, so it is a tone.

And the reverse trap: E to F is a semitone, even though both are white and they look like a normal white-key pair. So is B to C. Those are the two places in the octave where neighbouring letters have nothing between them.

Why scale formulas use them

The major scale is:

tone – tone – semitone – tone – tone – tone – semitone

Apply that from C and you land exactly on the white keys, because the two semitone steps in the pattern line up with the two natural semitones E–F and B–C. Apply it from anywhere else and you need black keys to keep the shape — which is where key signatures come from.

Twelve semitones per octave

Count from C up to the next C and you pass through twelve semitones. That is the octave, and it is why there are twelve pitches to work with — see octaves and register.