Intervals and chords

An interval is the distance between two notes. Count letter names, including both the start and end: C to E = C, D, E = a 3rd. C to G = a 5th. C to C = an octave (8th).

On the staff: a 3rd is line-to-next-line (or space-to-next-space); a 5th is line-to-line skipping one line; odd intervals stay on the same type (line/line), even intervals switch (line/space).

The number isn't the whole story. A 3rd can be major (4 half steps, C–E) or minor (3 half steps, C–E♭). 4ths, 5ths and octaves are usually perfect (C–F = 5 half steps, C–G = 7).

A triad is three notes stacked in 3rds: root, 3rd, 5th. On the staff it looks like a snowman — three lines in a row or three spaces in a row.

Major triad = major 3rd + minor 3rd on top (C–E–G). Minor triad = minor 3rd + major 3rd (A–C–E). Diminished = two minor 3rds (B–D–F).

Chords are named by the scale degree of their root: I, ii, iii, IV, V, vi, vii°. Upper case = major, lower case = minor.

I, IV and V are the three primary chords. In C major: C, F, G. A huge amount of music — hymns, folk, pop — uses mostly these three. V almost always wants to resolve to I.