Intervals

An interval is the distance between two pitches. It is the smallest unit of music theory that means anything on its own, and it is the layer everything above it is built from.

Two labels, not one

Every interval has a number and a quality.

The number counts letter names, inclusive: C up to E covers C, D, E, so it is a third. The number ignores sharps and flats entirely — C to E and C to E♭ are both thirds.

The quality — major, minor, perfect, augmented, diminished — measures the exact size in semitones. C to E is four semitones, a major third. C to E♭ is three, a minor third. What is an interval works through both labels properly.

Why bother

Because the layer above is entirely intervals:

• A major chord is a major third stacked with a minor third.
• A minor chord flips those two.
• A major scale is a fixed pattern of tones and semitones — a specific sequence of intervals.
• Key signatures exist to keep that pattern intact from any starting note.

Learn intervals and the rest stops being a list to memorise.

The full set of qualities

Seconds, thirds, sixths and sevenths are major or minor. Unisons, fourths, fifths and octaves are perfect. Any of them can be stretched or squashed into augmented or diminished.

Two further ideas make the system smaller than it looks: inversion, which pairs every interval with another so you only have to learn half of them, and compound intervals, which explains the numbers in chord symbols like C9 and Cm11.

Naming them by ear

Recognising a third by sight is a different skill from recognising it by ear, and the ear version is the one that makes you useful in a rehearsal room. That is ear training, and it is worth starting earlier than most people do.