A key is a scale treated as home. Rather than write a sharp or flat on every note, music puts them once, at the start of each line: the key signature.
Key signatures
Every major and minor key has a key signature of up to seven sharps or seven flats. Their order never changes, and it follows the circle of fifths.
| Order | |
|---|---|
| Sharps | F♯ C♯ G♯ D♯ A♯ E♯ B♯ |
| Flats | B♭ E♭ A♭ D♭ G♭ C♭ F♭ |
So a signature of two sharps is always F♯ and C♯: D major, or its relative minor, B minor. The flats are the sharps backwards. Keys with seven sharps or seven flats are usually written as their enharmonic equivalents instead.
The circle of fifths
Arrange the twelve pitch classes in rising perfect fifths and you get the circle of fifths: C at the top with no sharps or flats, then G, D, A and on, clockwise, each key adding one sharp. Read it counter-clockwise and it is a circle of fourths, each step adding a flat.
Neighbours on the circle share all but one note, which is why moving to the next key round, from C to G or C to F, sounds so natural. Keys across the circle from each other share the fewest.
One more fact, for lesson 12: twelve perfectly pure fifths of 3:2 overshoot the starting note slightly. The leftover is called the Pythagorean comma, and it is the reason keyboards are tuned in equal temperament.
Sources
Every date, name, tempo and origin on this page comes from these pages, read on 2026-10-24.