Circle of fifths

You can already name the chords of a key by number. This is where they live. Put the twelve major keys in a ring, each one a fifth above the last, and the I, the IV and the V of any key end up standing side by side, which is most of why those three keep turning up together in song after song.

Start at C on the diagram and walk clockwise. Every step rises by a fifth and adds one sharp to the key signature: C, then G, then D, then A. Walk the same circle the other way and every step rises by a fourth and adds a flat: C, then F, then B♭, then E♭. Nobody drew two diagrams. The circle of fourths and the circle of fifths are one wheel, and the name depends on which way you are walking.

Neighbors on the wheel sound related because they nearly are. Any two keys sitting side by side share six of their seven notes, so moving between them shifts one note and nothing else. That single fact is what the wheel is for. Put your key in the middle of three, and the neighbor counterclockwise is your IV chord and the one clockwise is your V, the chord that leans hardest back toward home.

Try it on C. Its counterclockwise neighbor is F and its clockwise neighbor is G, which gives you C, F and G: the three chords under a startling amount of what the room plays. Drop to the inner ring for the relative minor, Am, and you have C, Am, F and G, which is the doo-wop shape from the last article. Now use it the other way. If a song in C sits too high to sing, step one place clockwise to G and take the whole shape with you, and C, F and G becomes G, C and D.

You do not need any of this to turn up on a Saturday, and a Strum Leader will still show you the chords before each song. But the next time somebody calls a key you have not played in, you can count two places on the wheel and know what is coming.