Circle of Fifths

Physics: C→G→D→A→E→B→F#/Gb→Db→Ab→Eb→Bb→F→C: 12 perfect fifths nearly close the circle. Each step adds 1 sharp or removes 1 flat. Tritone substitution: keys opposite (6 apart) share 2 notes (C and F# both contain E♭/D#). Relative minor is a major sixth below (or minor third above): Am is relative of C major. Closely related keys (adjacent on circle) share most notes; distant keys share fewer. Bach's Well-Tempered Clavier (1722) demonstrated all 24 keys were usable in well-temperament.