A key is a home note plus the scale built on it: music in G major feels at home on G and uses the notes of the G major scale. There are twelve major keys, and some are much closer relatives than others. Line them up a 5th apart and a simple pattern appears: each step adds exactly one sharp or one flat. Bent round into a ring, that line is the circle of fifths. Let's find out why the pattern works, then see what the circle is good for.
Look at the steps of the : W W H, then W, then W W H again. It's two identical halves of four notes, joined by a whole step:
That's the key to the whole circle. The upper half of C major, G A B C, has exactly the shape a lower half needs. So it can start a new major scale: G major.
Walk up a 5th from C to G. G major keeps C major's upper half as its own lower half, and needs a new upper half. Built with the same W W H, that half forces one note up a : F becomes F♯. Walk down a 5th instead, from C to F, and C major's lower half becomes F major's upper half; the new lower half forces B down to B♭. Step around and see it happen every time:
C major: no sharps or flats
C major uses only the natural notes. Step up or down a 5th.
Notice the order the new sharps appear in: F♯, C♯, G♯, D♯ … each a 5th above the last. The flats come in the reverse order, B♭, E♭, A♭ … That's the order they're written in a key signature.
Twelve steps of a 5th visit all twelve notes and come back to C, so the keys form a circle: sharps clockwise, flats anticlockwise. At the bottom they meet: G♭ major (6 flats) and F♯ major (6 sharps) are the same notes, written two ways, and so are D♭ and C♯, B and C♭. It's the same circle that gave us the twelve notes in Why Twelve Notes?
The small label under each key is its key signature: 2♯ means two sharps, ♮ means none. Where a key can be written two ways, both are shown. The inner ring shows each major key's , which uses the same notes (more on that in Relative and Parallel Keys).
Neighbours on the circle differ by just one note, so music moves between them smoothly. Keys on opposite sides share the fewest notes and sound furthest apart.
A quick preview of the Chords chapter: a chord is several notes played together, and you can build one on every note of a scale. Musicians number them with Roman numerals: I is the chord on the 1st note, IV on the 4th, V on the 5th. Capitals are major chords, small letters (ii, vi) minor ones. A chord symbol like Dm means D minor; a letter on its own, like F, is a major chord.
On the circle, a key's three major chords are neighbours: I in the middle, IV one step anticlockwise, V one step clockwise. Its three minor chords sit right inside them. (The chord on the 7th note, vii°, is neither major nor minor, so it has no place on the circle.) Pick a key's root note:
A chord progression is a series of chords played one after another. Many progressions step anticlockwise, each chord a 5th below the last, because the pull from a chord down a 5th is the strongest there is. That's why V, the chord on the 5th note, pulls so strongly back to I. Listen to vi–ii–V–I in C: Am, Dm, G, C. Their A, D, G, C step three times round the circle, back home to C.
Explore every key, its signature and its scale in the interactive circle.
Next: Key Signatures