Diatonic chords: harmonising the scale

Chords grow out of the scale

If we build a triad on each degree of the scale (root + third + fifth, all from the same scale), we get the diatonic chords of the key. These are seven chords that naturally belong together - which is why a song in one key sounds coherent.

Roman numerals

We label chords by degree with Roman numerals. Upper case means a major chord, lower case minor, and the ° sign diminished:

• I - major (the root, the tonic)
• ii - minor
• iii - minor
• IV - major (subdominant)
• V - major (dominant)
• vi - minor (the relative minor)
• vii° - diminished

Example in C major

In the key of C major the diatonic chords are: C - Dm - Em - F - G - Am - B°. These are the chords you will hear in countless songs in C major. Every major key follows the same quality pattern (major, minor, minor, major, major, minor, diminished).

Why this is useful

Once you know a key's diatonic chords you can: quickly find chords that will sound good together; understand which chord 'does not belong' (and use it for surprise); and recognise progressions in Roman numerals that work in any key.

Three degrees stand out: I, IV and V (in C major: C, F, G). Together they contain all seven tones of the scale and form the backbone of countless songs. More on them in the article on chord progressions.