Semitone

On a piano, C to C# is a semitone, and so is C# to D. Most pairs of neighboring white keys have a black key between them, but two pairs do not: E to F and B to C are semitones with nothing in between. Twelve semitones make an octave, so C up to the next C is 12 semitones.

The major and minor scales are patterns of whole and half steps. The major scale runs whole, whole, half, whole, whole, whole, half. In C major:

• C to D, D to E: whole steps
• E to F: half step
• F to G, G to A, A to B: whole steps
• B to C: half step

Natural minor uses the same white keys from A, which gives a different pattern: whole, half, whole, whole, half, whole, whole (A B C D E F G A). Some scales use wider gaps too: harmonic minor has three semitones between F and G#.

The pattern is where sharps and flats come from. Start the major pattern on G and the step up from E has to be a whole step, so F becomes F#: G A B C D E F# G. That single sharp is the G major key signature. Start it on F and the fourth note has to sit a half step above A, so B becomes Bb.

Two notes a semitone apart, played together, rub against each other, and that rub is what most people mean by a wrong note. The major pentatonic scale has no half steps at all. It drops the two major-scale notes that sit a half step from a neighbor, so C major pentatonic is C D E G A, with no F and no B. Why the pentatonic scale sounds good explains the rest.

Synths count pitch in semitones too. Transpose and pitch-bend ranges are set in semitones, and fine tuning uses cents, where 100 cents make one semitone. Two oscillators 10 cents apart, a tenth of a semitone, sound like one thicker note: that is detune. In Gelato Synth, the Detune slider on Osc B goes up to 50 cents either way, half a semitone.

Every key page on this site lists its whole and half steps. The G major page shows the F# example above.