Key Signatures & the Circle of Fifths
The sharps or flats at the start of the line name the key. Their order never changes, and arranged in a circle they map every key's neighbours.
Builds on: 2.1 The Major Scale2.2 Minor Scales
Declare it once
A piece in D major needs F♯ and C♯ in every bar. Rather than writing a sharp before every F and C, the composer writes them once, after the clef, as the key signature: those sharps apply to every F and every C on the page, in every octave, until cancelled by a natural sign or a new signature. The signature is a promise about which seven notes the piece lives in — its key. Read it and you know the scale before you hear a note.
The order is fixed
Sharps always enter in the same order: F C G D A E B. One sharp is always F♯; two are F♯ and C♯; three add G♯. Flats enter in the reverse order — B E A D G C F — one flat is always B♭, two adds E♭. Each order is a chain of fifths (F to C is a fifth up, C to G, and so on) and each has its sentence:
Reading the key off the signature
Two rules of thumb make any signature instant. Sharps: the key is a half step above the last sharp — three sharps end on G♯, so the key is A major. Flats: the key is the second-to-last flat — three flats are B♭ E♭ A♭, so the key is E♭ major. (One flat has no second-to-last; that is F major, the one to memorise.) For the minor key with the same signature, drop three semitones from the major tonic: three sharps is A major or F♯ minor.
The circle of fifths
Arrange the keys so that each step clockwise goes up a fifth: C, G, D, A, E, B, F♯. Each step adds one sharp. Go the other way — C, F, B♭, E♭, A♭, D♭, G♭ — and each step adds a flat. The two directions meet at the bottom, where F♯ major (six sharps) and G♭ major (six flats) are the same twelve keys spelled two ways: enharmonic keys. Draw the relative minors on an inner ring and the diagram becomes the circle of fifths, the single most useful picture in music theory. Neighbouring keys share six of their seven notes, which is why modulating to the dominant or subdominant sounds smooth (Unit 5) and why the chords of a song tend to cluster on one side of the circle.
One sharp is G major or E minor. To decide, look at where the melody comes to rest and whether the seventh is raised: a stray D♯ in a one-sharp piece is the harmonic minor’s leading tone, and the key is E minor.
The lab is the circle itself. Click any key to hear its scale, see its signature on a grand staff and read off its relative minor and its neighbours. The full table of keys with memory hooks lives on the reference page.
🎼 Lab — The Circle of Fifths
Twelve major keys around the outside, their relative minors inside. Click a key to hear its scale and see its signature on the staff below; click a minor to hear the minor scale on the same notes.
- Walk clockwise from C: one sharp appears per step, always the next letter of F C G D A E B.
- Walk counter-clockwise: the flats arrive as B E A D G C F.
- Meet at the bottom: F♯ major and G♭ major sound identical — six sharps or six flats, your choice.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.