Key Signatures

Every scale beyond C major and A minor needs accidentals to keep the right pattern of tones and semitones. Instead of repeating those accidentals on every note, there is a smarter way: the key signature.

01What a key signature is

The key signature is a set of accidentals written right after the clef, at the start of every line of the staff. It declares that certain notes are always played raised or lowered by a semitone, for the whole piece, unless a natural sign (♮) temporarily restores them.

In C major you need no accidentals, so the key signature is empty. In G major you need an F♯ on every F in the score. Instead of writing the ♯ every time, you put it once in the key signature.

02The order of sharps and flats

Accidentals don't enter the key signature in random order. The sharps always follow this order:

F · C · G · D · A · E · B

The flats follow the reverse order:

B · E · A · D · G · C · F

Every key uses the first accidentals of this order, as many as it needs. The more sharps (or flats) a key signature has, the further we go down the order. G major has only the first sharp, F♯. D major has the first two, F♯ and C♯. A major the first three, F♯ C♯ G♯, and so on. The same happens with the flats: F major has only B♭, B♭ major the first two, B♭ and E♭.

03Relative scales, same signature

Two scales, one major and one minor, that use exactly the same notes are called relatives. Every major scale has its relative minor a step and a half below (a minor third). Because they share the same notes, they also share the same key signature. C major and A minor are relatives and have no accidentals. G major and E minor are relatives too, with 1 sharp (F♯).

To tell which of the two scales is being played, you look at the beginning and end of the piece and at the note that "feels" like home. The key signature alone doesn't tell you whether the piece is in the major or the minor.

It is also worth recalling something from the lesson on tones and semitones. In every scale, each note name appears only once. In G major, for example, there is already a G (the tonic). That is why we write the seventh degree as F♯ and not G♭, even though they sound the same: written as G♭, we would have two notes named G. So the key signature always uses the spelling that gives each degree its own name.