The circle of fifths for mandolin

What the circle of fifths is

Start on C, which has no sharps or flats. Go up a fifth to G and the key has one sharp. Up another fifth to D and it has two. Keep going and each step adds a sharp, until you come round through the flat keys and back to C. That is the whole diagram: the twelve keys laid out so that the ones next to each other share most of their notes.

Two things ride along on it. Every major key has a relative minor that uses the same notes, and it sits inside the wedge on this chart (A minor under C, E minor under G). And any key's two nearest neighbors are its IV and V chords, which is most of the chords in most of the tunes you will ever play.

Every key on the wheel

Why it is easy on a mandolin

A mandolin is tuned in fifths: G, D, A, E from the low course up. Those four notes are four spokes in a row on the wheel. The instrument is the circle, or a quarter of it, already in your hands.

Play in D and the strings either side of your D course are G and A, which are D's IV and V. Play in A and they are D and E. This is why bluegrass lives in G, D and A. Between those three keys and their IV and V chords, every root but C is an open string. It is also why a fiddle tune in B flat feels like a foreign country: none of its chords sit on the open strings.

How to practice the circle of fifths

You can read the wheel off the paper for years and still not have it. Two exercises put it in your head.

Before you play any tune, say its key and its signature out loud. Salt Creek in A: three sharps, F, C and G. Then say its IV and V. Do this every time and the wheel is in your head within a couple of weeks.

Then play the neighbors. Take a key, play its I, IV and V with a band, then move one spoke clockwise and do it again. The jam builder below is set up for this: it opens on G with C and D, and picking another key moves every chord with it. Go all the way round at whatever tempo is honest. The keys with four and five sharps will be slow. That is the point.

The Keys lessons in the app go through all of this and then listen while you play it.

Why it matters

At a jam nobody hands you a chart. Somebody says "Red Haired Boy in A" and the band starts. If you know the circle, the chords are already in your hand: A, D, E, and the G the tune leans on. If you do not, you are watching the guitar player's left hand for the whole tune.

It also gets a tune into a singer's key. A song learned in G that needs to be in A moves every chord one spoke clockwise. Same chord shapes, two frets up the neck.