The Chords in a Key

Songs do not pick chords at random, and the reason is simple once you see it: build a chord on each note of a scale, using only notes from that scale, and you get seven chords that belong together. Those are the chords of the key.

Do it in G major, whose notes are G A B C D E F sharp, and you get G, A minor, B minor, C, D, E minor, and a strange one on F sharp. Notice the pattern of qualities — major, minor, minor, major, major, minor. That order is the same in every major key, always, because it comes from the scale's step pattern rather than from the particular notes.

Because the pattern is fixed, musicians number the chords instead of naming them. Roman numerals, with capitals for major and lower case for minor: I, ii, iii, IV, V, vi. In G that means G is I, A minor is ii, C is IV, D is V, E minor is vi.

This is the single most useful idea in this path. A progression described as I V vi IV is the same progression in every key, so once you recognise it you have recognised it everywhere.

G

G major, the I chord of this key.

Em

E minor, the vi chord — same key, same notes, different starting point.

This track is in G major and uses G, A minor, C, D and E minor — the I, ii, IV, V and vi of the key, and not one chord from outside it.

Watch the changes and say the numeral rather than the letter as each chord arrives: one, two, four, five. It feels clumsy for about a minute and then it starts doing your thinking for you.

The reason to bother is transposition. If you know a song is I vi IV V, you can play it in any key the singer needs without relearning anything.

G major, entirely diatonic. Say the numeral, not the letter.