Semitone
A semitone, also called a half step, is the smallest interval in Western music, the distance from one note to the very next one, such as E to F or C to C♯.
What does a semitone look like on real notes?
On a piano, a semitone is any key to the key right next to it, black or white: C to C♯, C♯ to D, E to F. On a guitar it is one fret.
Two semitones make a tone, also called a whole step: C to D, F to G. Twelve semitones make an octave, and every interval is measured by how many it spans. A major third is 4, a perfect fifth is 7.
Among the white keys, only two pairs are a semitone apart: E–F and B–C. Every other pair of white neighbours has a black key between them, so they are a tone apart.
In modern equal temperament, all twelve semitones are the same size: each one raises the frequency by the same ratio, the twelfth root of 2, about 1.0595.
How does a semitone sound?
Close, and pulling. Two notes a semitone apart, played one after the other, sound like one note leaning on the next. Played together, they clash.
You know it from famous openings. The Jaws theme alternates two notes a semitone apart, E and F. Beethoven’s Für Elise opens by rocking between E and D♯.
The semitone is also what makes music want to resolve. The leading tone sits a semitone below the tonic, and a single semitone is the whole difference between a major chord and a minor one.
What is the most common mistake with semitones?
Assuming neighbouring white keys are always a tone apart. E–F and B–C are not, and that is why the major scale has its two semitone steps exactly there.
The other is vocabulary. Semitone (UK), half step (US) and half tone name the same distance. Tone and whole step name double that.
Try it: Notes and Semitones has you count semitones on the keyboard to build chords from any root.