Key Signature
The set of sharps or flats a piece carries, declared once at the start, that pins the music to a particular key and its home note.
Key Signature
A key signature is the small cluster of sharps or flats printed at the very start of a staff. It is a standing instruction: "for the whole piece, play these notes raised or lowered." More than mere notation, it announces the music's key — its home note and the Scale gathered around it.
One scale, written once
Every major Scale except C uses some black keys. Rather than mark every single one with an accidental, we declare them up front. The key of G major needs an F♯ throughout, so its signature is one sharp. D major needs F♯ and C♯ — two sharps. F major needs B♭ — one flat.
| Key | Signature | Altered notes |
|---|---|---|
| C major | — | none |
| G major | 1 sharp | F♯ |
| D major | 2 sharps | F♯, C♯ |
| F major | 1 flat | B♭ |
| B♭ major | 2 flats | B♭, E♭ |
The order in which sharps appear is itself fixed — F♯, C♯, G♯, D♯, … — and that order is no accident: each new sharp is a fifth above the last, which is precisely the pattern charted by the Circle of Fifths.
Counting sharps and flats
There is a tidy shortcut hidden in the Circle of Fifths: the number of sharps in a key equals how many steps clockwise it sits from C, and the number of flats equals how many steps counter-clockwise. So a key signature is really just a position on that circle, translated into ink on the page.