How diatonic triads work

As we’ve seen, triads are three-note chords made up of the lowest note (known as the root note), a middle note, (the third), and the top note (the fifth). But when you play a set of diatonic chords like this, built using notes that all belong to a major scale, why do some of them end up being minor chords instead of major?

A major scale is major due to the set pattern of intervals between its notes, or ‘degrees’. The interval between the first two degrees, C and D, is a whole tone, between the next two, D and E, is another whole tone, but between E and F the interval is a semitone. Going on, if T stands for whole tone and S for semitone, the full intervallic pattern for a major scale is T-T-S-T-T-T-S.

A chord’s major or minor-ness is known as its quality, and chord quality depends on the number of semitones or ‘interval’ between the third and root note. Diatonic chords mean you can only use scale tones to form them, so as the scale has a set intervallic pattern, the interval between the root note and the third will vary from chord to chord. So, with a C major chord, you have C as the root and E as the third, a distance of four semitones. This equates to a major third interval, which gives a C major chord its major quality. Go to the next note in the C major scale, however – D – and form the diatonic D chord using alternate notes from the scale, and the distance between its D root and F third reduces to three semitones. This is a minor third interval, which in turn makes the diatonic D chord in the key of C major a minor chord – D minor.

Continue on up the scale playing diatonic triads, and you’ll find that the irregular intervallic pattern produces a mixed bag of major and minor chords (plus one diminished chord), as the interval between each chord’s root and third changes as you go up.