The circle of fifths arranges the keys in a chain of perfect fifths: C → G → D → A… After twelve steps the circle closes, returning to the starting note. Each clockwise step adds one sharp, each counter-clockwise step one flat.
It is not merely a mnemonic for key signatures but a map of kinship: neighbouring keys differ by only one accidental and pass easily into one another, while opposite keys (e.g. C major and F♯ major) are the most remote. Composers use the circle of fifths as a compass for modulation, and jazz musicians for building progressions (the famous ii–V–I turnaround moves precisely by fifths). Understanding the circle of fifths reveals the inner logic of all tonal music.
