An interval is a musical distance measured in semitones, and it repeats every octave (12 semitones)

An interval is simply the distance between two notes, measurable in semitones (half steps). The smallest step is the semitone; two semitones make a whole step (whole tone). The octave is 12 semitones and is the same note higher — moving up 12 semitones lands on the same letter. Because of this, intervals wider than an octave repeat the pattern of the intervals within it: a ninth behaves like a second, a tenth like a third, and so on. This is why interval theory usually stops at the octave. Two identical notes are a unison, a distance of zero. These semitone counts are the raw measurement that number names (second, third…) and qualities (major, perfect…) are then layered onto.

Examples

Fret 5 A up to fret 17 A = 12 semitones = one octave, the same note higher. A ninth spans an octave plus a second and is functionally the same scale note as the second.

Assessment

State how many semitones are in a semitone, a whole tone, and an octave. Explain why theory books usually stop naming intervals at the octave rather than continuing past it.