Intervals: Number & Quality

The distance between two notes is called an interval, and we look at its “number” first: count the two note names (including the starting one) up the scale one by one, and whatever you count to is the number. C to C is a unison (the same note), C to D a 2nd, C to E a 3rd, C to G a 5th… and a full octave is, fittingly, an 8th.

The number alone isn't enough: a third that spans 4 semitones is a major third (bright), while one that spans 3 semitones is a minor third (soft and dark) — and that's where major and minor chords come from. The 2nd, 3rd, 6th and 7th come in major and minor; the unison, 4th, 5th and octave are called “perfect” (the most stable, the most consonant).

On the guitar it's very visual: on a single string, each fret apart is a semitone, so a major third = 4 frets, a minor third = 3 frets, a perfect fifth = 7 frets.

Intervals aren't only calculated — they're heard: the perfect fourth and perfect fifth feel the most “stable, open, comfortable,” the major and minor thirds and sixths feel “sweet,” the major second feels a bit crowded, and the minor second and major seventh feel the most “tense and grating.” Sound two notes together, guess by ear first, then check your answer.

Here's another humble trick I tell my students over and over: sing the notes out as you play them. Sing them enough, and the next time you hear that same relationship your ears will react before your brain does — that's how a feel for intervals grows.

A triad = a root with a third stacked on, then another third; a dominant seventh chord = a minor third stacked on top of that triad; and in improvising, the “leading tone” and “target notes” are located by interval too. Once you can measure intervals, the chord construction you've learned and the secondary dominants and improvising still to come all link up.