The octave is divided into 12 equal semitones, giving 12 distinct pitch classes

Although ‘octave’ implies eight and the seven letter names A–G repeat at the octave, most modern instruments divide the octave into twelve equal smallest steps, not eight. That smallest step is the semitone (semi = half); two semitones make a tone. Counting up chromatically from any note, you pass through twelve semitone steps before the note name repeats an octave higher — so there are twelve distinct pitch classes per octave. This is the same on all instruments: on a piano the twelve include white and black keys; on a guitar they are twelve frets. The semitone is the smallest working distance between two notes, and every interval, scale and chord is measured in these semitone steps.

Examples

From A, counting semitones 1–12 (white and black keys on piano, or twelve guitar frets) brings you back to A an octave higher. C up to the next D is two semitones = one tone = an interval of a second. The gap C to the very next key is one semitone.

Assessment

State how many semitones make an octave and how many make a tone. Given a starting note on a keyboard, count up a specified number of semitones and name where you land, showing that twelve returns you to the same pitch class.