Key signatures

A key signature is the group of sharps or flats printed after the clef, at the start of every line. It applies to every octave, for the whole piece, until it is cancelled or changed.

That distinguishes it from an accidental in the music, which applies to one octave for one bar.

The order never varies

Sharps always appear in this order:

F♯ C♯ G♯ D♯ A♯ E♯ B♯

Flats appear in exactly the reverse:

B♭ E♭ A♭ D♭ G♭ C♭ F♭

A key signature with three sharps is always F♯, C♯, G♯ — never some other three. This is not arbitrary. Each is a fifth above the last, which is what the circle of fifths draws.

Naming a major key from its signature

For sharps: take the last sharp and go up one semitone.

Three sharps — last is G♯ — one semitone up is A. The key is A major.

For flats: the key is the second-to-last flat.

Three flats — B♭, E♭, A♭ — the second-to-last is E♭. The key is E♭ major.

Two exceptions to memorise: no accidentals is C major, and one flat is F major (there is no second-to-last flat to point at).

The full list

Major or minor?

A key signature does not tell you. Each one serves a major key and its relative minor, which share the same notes.

To decide, look at the music rather than the signature:

• Which note does the piece end on? The final bass note is the strongest clue there is.
• Are there raised sevenths? A piece in A minor that keeps writing G♯ as an accidental is announcing itself — that is harmonic minor at work.
• Which chord does it keep returning to?

Why signatures exist at all

Purely compression. A major scale starting on any note but C needs accidentals to keep its tone–semitone pattern intact. Rather than mark them on every note, notation declares them once and moves on.