Circle of fifths

What it's for

The circle of fifths is the key tool of music theory: it shows keys, their accidentals (sharps/flats) and the «family» chords that sound good together.

How to use it

Once you pick a key, you see the key signature, the relative minor (sharing the same notes) and the I-ii-iii-IV-V-vi degrees, the most-used chords in that key.

Why it is a circle

Going up a fifth from a note and carrying on — C, G, D, A, E… — after twelve steps you are back at the C you started from. It is not a coincidence: twelve fifths and seven octaves cover almost exactly the same distance, and equal temperament rounds that «almost» away until the circle closes. It is the same rounding that lets a piano play in every key.

The scale gains at each clockwise step one sharp, at each anticlockwise step one flat, and the accidentals always appear in the same order. That is why a key signature can be read at a glance: counting the sharps or flats tells you the key at once.

Keys that resemble each other sit close together

Two adjacent keys on the circle share six notes out of seven, and that explains most of popular music. Modulating to the neighbouring key sounds natural, because a single note changes; jumping to the other side of the circle sounds like a tear, and that is an effect used on purpose.

Hence also the choice of chords: the three most used major chords of a key are it and its two neighbours on the circle, that is the first, fourth and fifth degrees. C, F and G are adjacent, which is why thousands of songs use exactly those three.

The relative minor, and why it is not another key

Every major key has a relative minor that uses the very same notes, three semitones lower: C major and A minor both have no accidentals. What changes is not the notes but the centre of gravity, that is the note the music settles on.

It is why a song can turn from cheerful to wistful without anyone changing key: it is enough to end the phrases on A instead of C. And it explains why a key signature alone does not say whether a piece is major or minor: you have to look at how it begins and above all at how it ends.

What it is really for, in practice

For transposing: knowing the circle is symmetrical, you move every chord by the same number of steps. For improvising: the notes of the key are the ones with no accidentals against the signature, and neighbouring keys offer the extra note that colours a phrase.

And for writing a chord sequence that works: the strongest movement in Western harmony is anticlockwise, that is by descending fifths, and the turnaround that closes almost every piece is exactly that. The circle is not a table to memorise, it is a map that says which steps sound near and which sound far.

Nearby tools

The notes of each scale are in Musical scales, those of each chord in Chord notes. To build a sequence there is Chord progressions, to change key Transpose chords and for distances Musical intervals.