Music Theory Reference

The circle of fifths arranges the 12 keys in a ring, each one a perfect fifth (7 semitones) above the last. C at the top, G to the right, D below G, then A, E, B, F#, and round through the flats. Each step clockwise adds one sharp to the key signature. Each step anticlockwise adds one flat. So G has 1 sharp (F#), D has 2 sharps (F#, C#), A has 3 (F#, C#, G#), and so on.

Why this matters in practice: keys that are next to each other on the circle sound related, so a song in C Major can naturally move to G Major or F Major without sounding jarring. Keys on opposite sides of the circle (C and F#, for example) sound distant and feel like a hard left turn. When you are writing a bridge or a key change, the circle tells you which target keys will sound smooth. Most pop key changes go up by a semitone or up a 4th; both are easy because you are travelling one or two steps round the circle.

Every major key has a relative minor that uses exactly the same notes; you just start the scale from a different place. C Major and A Minor share the same 7 notes (C, D, E, F, G, A, B); the difference is which note feels like home. Start a melody on C and resolve to C and the song feels major. Start on A and resolve to A and the same notes feel minor. This is why pop songs often shift between, say, C and Am verse-to-chorus without a key change.

The reference also covers the modes (Dorian, Phrygian, Lydian, Mixolydian, Locrian) which are essentially the major scale started from each of its other 6 notes. Mixolydian (the 5th mode) gives you the dominant-7 sound used in blues and funk. Dorian (the 2nd mode) gives you the cool, jazzy minor used in songs like So What or Scarborough Fair. The [scale finder](/scale-finder) lets you hear each mode and see it on a guitar fretboard so the abstract names connect to real shapes.

An interval is the distance between two notes, measured in semitones. The reference lists all 13 intervals from unison (0 semitones) to octave (12). A perfect 5th (7 semitones) is the most consonant interval after unison and octave, which is why it is the foundation of every power chord and the structural skeleton of Western harmony. A tritone (6 semitones) is the most dissonant; it was historically called the 'devil's interval' because it sounds so unstable.

You can hear interval characters in songs you already know. The first two notes of Twinkle Twinkle are a perfect 5th (C to G). Somewhere Over the Rainbow opens with an octave leap. The Star Wars theme starts with a perfect 5th (C to G in the original key). The Simpsons theme opens with a tritone (C to F#). Once you can hear these, you can name any interval by ear, which is the foundation of relative pitch. The [chord library](/chord-library) shows the intervals inside each chord type.

In any major key, the seven chords built from the scale follow a fixed pattern: I (Major), ii (minor), iii (minor), IV (Major), V (Major), vi (minor), vii (diminished). In C Major, that gives you C, Dm, Em, F, G, Am, Bdim. This is the menu of chords that fit naturally in C; pick from this menu and the song will sound cohesive.

The Roman numerals matter because they let you write chord progressions that are key-independent. I-V-vi-IV is the 'four chord' progression that powers thousands of pop songs (Let It Be, With or Without You, Don't Stop Believin'). In C it is C-G-Am-F. In G it is G-D-Em-C. In D it is D-A-Bm-G. Once you memorise the pattern as numerals you can transpose any song to any key by reading off the right notes from the major scale of that key.