Keys, key signatures and the circle of fifths

What a key is

A key tells you which root tone and which scale a piece is built around. The key of C major uses the tones of the C major scale, the key of G major the tones of the G major scale, and so on. The root is 'home' - the melody likes to return there.

Key signatures

So we don't have to write a sharp or flat before every note, we gather them at the start of the staff as a key signature. G major, for instance, has one sharp (F♯), F major one flat (B♭). The signature tells you at a glance which key you are in.

The circle of fifths

The circle of fifths is a diagram that arranges all twelve keys by perfect fifths. Each step clockwise (up a fifth) adds one sharp; each step counter-clockwise (down a fifth) adds one flat:

• Up in fifths: C → G → D → A → E → B (more and more sharps)
• Down in fifths: C → F → B♭ → E♭ → A♭ (more and more flats)

The order of accidentals

Sharps are always added in the order F-C-G-D-A-E-B, flats in the reverse B-E-A-D-G-C-F. This consistent pattern follows from the structure of the circle of fifths.

The relative minor

Every major key has a relative minor with the same key signature, sitting a minor third lower: C major and A minor, G major and E minor. That is why pieces in relative keys look identically written yet sound different.