Why guitarists should learn theory
Theory is a map of why music sounds the way it does. Instead of memorizing hundreds of shapes, you learn a few ideas that explain all of them, and improvising, songwriting and learning songs by ear all get easier.
Intervals on guitar
An interval is the distance between two notes. On one string every fret is one semitone, so intervals are easy to see and to measure:
| Interval | Semitones (frets) | From C |
|---|---|---|
| Minor 2nd | 1 | C to D♭ |
| Major 2nd | 2 | C to D |
| Minor 3rd | 3 | C to E♭ |
| Major 3rd | 4 | C to E |
| Perfect 4th | 5 | C to F |
| Tritone | 6 | C to F♯ |
| Perfect 5th | 7 | C to G |
| Minor 6th | 8 | C to A♭ |
| Major 6th | 9 | C to A |
| Minor 7th | 10 | C to B♭ |
| Major 7th | 11 | C to B |
| Octave | 12 | C to C |
How chords are built
A triad is three notes stacked in thirds.
- Major: root, major 3rd, perfect 5th (1, 3, 5). C major is C, E, G.
- Minor: root, minor 3rd, perfect 5th (1, ♭3, 5). A minor is A, C, E.
- Diminished: 1, ♭3, ♭5. B diminished is B, D, F.
- Dominant 7th: a major triad plus a minor 7th (1, 3, 5, ♭7). G7 is G, B, D, F.
The chords in a major key
Build a triad on every note of the major scale and you get the same pattern in every key: major, minor, minor, major, major, minor, diminished. In Roman numerals: I, ii, iii, IV, V, vi, vii°.
| Key | I | ii | iii | IV | V | vi | vii° |
|---|---|---|---|---|---|---|---|
| C | C | Dm | Em | F | G | Am | B° |
| G | G | Am | Bm | C | D | Em | F♯° |
| D | D | Em | F♯m | G | A | Bm | C♯° |
| A | A | Bm | C♯m | D | E | F♯m | G♯° |
| E | E | F♯m | G♯m | A | B | C♯m | D♯° |
Chord tones: the secret of melodic solos
The notes of the chord being played (its root, 3rd and 5th) are its chord tones. Landing on a chord tone when the chord changes makes a solo sound connected to the music. Over A minor, aim for A, C or E; when the song moves to D minor, aim for D, F or A.
The CAGED system
The five open chord shapes C, A, G, E and D can be moved up the neck to play any major chord in five places, and in that order (C, A, G, E, D) they link together along the neck. For example, the E shape barred at the 3rd fret is G major, and the A shape barred at the 3rd fret is C major. CAGED lets you see chords, scales and arpeggios as one connected map of the fretboard.
Next: the circle of fifths, chord progressions and the notes on the fretboard.