Diatonic Chords Explained

How diatonic chords are built

To build a diatonic chord, you start on any note of the scale and stack every other note above it. Start on the 1st degree, add the 3rd, add the 5th. That gives you the I chord. Start on the 2nd degree, add the 4th, add the 6th. That gives you the ii chord. Repeat for all seven degrees and you have all seven diatonic chords.

Because the scale has a fixed interval pattern, the quality of each chord (major, minor, or diminished) is determined by where it falls in the scale. It is not something you choose. The structure of the scale decides it.

The seven diatonic chords in a major key

The pattern is: I major, ii minor, iii minor, IV major, V major, vi minor, vii diminished. This is the same in every major key. The chord names change but the quality pattern never does.

Roman numeral notation

Capital Roman numerals (I, IV, V) indicate major chords. Lowercase (ii, iii, vi) indicate minor chords. The diminished chord uses a degree symbol (vii°). This notation is key-agnostic, meaning I-IV-V describes the same harmonic relationship whether you are in A major, C major, or G major.

When someone says a song uses a I-IV-V progression, they mean the root chord, the chord built on the 4th degree, and the chord built on the 5th degree of whatever key the song is in. In G major that is G, C, D. In A major it is A, D, E.

Why the I, IV, and V dominate

The I, IV, and V chords are all major in a major key. They are the three brightest, most stable chords in the key. The V chord has a particular tension because of its leading tone (the 7th of the scale), which resolves naturally to the I chord. This V-I resolution is the strongest harmonic movement in Western music and the reason so many songs end with it.

The circle of fifths shows why: I, IV, and V are the three keys closest to each other on the circle, one step apart in either direction. That proximity means they share almost all their notes, so moving between them sounds smooth and natural.

Diatonic chords in a minor key

The natural minor scale also produces seven diatonic chords. The pattern is different: i minor, ii diminished, bIII major, iv minor, v minor, bVI major, bVII major. The i, iv, and v are all minor, which is why minor key progressions sound darker.

Common minor key progressions you will recognize: i-bVI-bVII (Am-F-G), i-iv-v (Am-Dm-Em), i-bVII-bVI-V (Am-G-F-E). Understanding what makes a chord major or minor is the foundation for reading these.

Diatonic chords and scales

Diatonic chords and scales come from the same source. If you are soloing over a diatonic chord progression, you can use the key's scale for the entire progression without switching scales. The scale fits all the chords because all the chords come from the scale.

This is why the minor pentatonic works over an Am-G-F-C progression. All four chords are diatonic to C major (or A natural minor). The A minor pentatonic is a subset of those same notes. For more on connecting scales to specific chords see what scales to play over chords.