Music Theory Basics: Intervals and Chords

You can play piano for years without theory, but a little of it makes everything faster: reading, memorising, transposing, and understanding why a chord progression works.

Intervals

An interval is the distance between two notes, counted inclusively by letter name.

C to E: C-D-E is three letters, so it is a third. C to G: C-D-E-F-G is five, so a fifth.

The number is only half the information. The exact width gives the quality:

| Interval | Semitones | Example |

|---|---|---|

| Unison | 0 | C–C |

| Minor 2nd | 1 | C–Db |

| Major 2nd | 2 | C–D |

| Minor 3rd | 3 | C–Eb |

| Major 3rd | 4 | C–E |

| Perfect 4th | 5 | C–F |

| Perfect 5th | 7 | C–G |

| Minor 6th | 8 | C–Ab |

| Major 6th | 9 | C–A |

| Minor 7th | 10 | C–Bb |

| Major 7th | 11 | C–B |

| Octave | 12 | C–C |

Perfect intervals (unison, 4th, 5th, octave) stay perfect — widen them by a semitone and they become augmented, narrow them and they become diminished. The others come in major and minor flavours.

Why "C to E" is a major third but "C to Eb" is minor: both span three letter names, but the semitone counts differ.

Building triads

A triad is two stacked thirds. Take a root, add the note a third above, add another third.

C major: C–E–G — root, major third (4 semitones up), perfect fifth (7 up).

Change the middle note and the quality changes:

• Major: root, +4, +7 → C–E–G
• Minor: root, +3, +7 → C–Eb–G
• Diminished: root, +3, +6 → C–Eb–Gb
• Augmented: root, +4, +8 → C–E–G#

Major sounds bright or stable; minor darker or sadder; diminished tense and unstable.

Seventh chords

Add another third on top:

• Cmaj7: C–E–G–B
• C7 (dominant): C–E–G–Bb
• Cm7: C–Eb–G–Bb
• Cm7b5 (half-diminished): C–Eb–Gb–Bb

Dominant sevenths want to resolve — that tension and release drives an enormous amount of Western music.

Chords in a key

In any major key, triads built on each scale degree follow a fixed pattern:

I (major), ii (minor), iii (minor), IV (major), V (major), vi (minor), vii° (diminished)

In C major that is C, Dm, Em, F, G, Am, Bdim.

This is why certain progressions appear everywhere — I–V–vi–IV in countless pop songs, ii–V–I throughout jazz. Once you know the pattern, you can play in any key without memorising each chord separately.

Inversions

A chord does not have to have its root at the bottom:

• Root position: C–E–G
• First inversion: E–G–C
• Second inversion: G–C–E

Inversions make bass lines move smoothly instead of jumping, and they are why left-hand accompaniment patterns look the way they do.

Chord symbols vs notation

Chord symbols (C, Am7, F#m) specify pitches but not arrangement or voicing — you choose how to voice them. Notation specifies exactly what to play. Lead sheets use symbols and expect you to fill in the rest; classical scores write it out.

Why this helps in practice

• Memorising becomes remembering a progression (I–V–vi–IV) rather than hundreds of individual notes
• Reading gets faster because you recognise chord shapes rather than counting letters
• Transposing becomes moving the pattern rather than re-deriving every note
• Improvising becomes possible once you know which notes fit