Diatonic Chords: The Harmony Inside a Key

Diatonic means "belonging to the key." Diatonic chords are the chords you can build using only the notes of a scale. No sharps or flats are added. Every note in every chord comes from the same seven-note pool.

In the previous article, you learned that chords are built by stacking thirds. When you apply this process to each degree of a major scale, you get seven chords. These seven chords are the harmonic foundation of the key.

This is not arbitrary music theory. These are the chords that sound like they belong together. When you hear a song and the chords feel natural, they are almost certainly diatonic to the key. Understanding this pattern lets you predict chord progressions and know which chords to reach for when writing.

Take the C major scale: C - D - E - F - G - A - B. Now build a triad starting on each note, using only notes from the scale.

Notice that you did not choose whether each chord is major or minor. The scale determined it. The intervals between scale notes dictate the chord qualities. This is why diatonic chords follow a predictable pattern.

The pattern of chord qualities is the same in every major key. Memorize this pattern once, and you know the diatonic chords for all twelve keys.

Roman numerals indicate the scale degree. Uppercase numerals mean major chords. Lowercase numerals mean minor chords. The ° symbol indicates diminished.

Three major chords (I, IV, V), three minor chords (ii, iii, vi), and one diminished chord (vii°). This 3-3-1 pattern is fundamental to Western music.

In the previous article, you learned that the interval from root to 3rd determines major or minor quality. A major third (4 semitones) creates major. A minor third (3 semitones) creates minor.

The major scale has half steps between degrees 3-4 and 7-1. These half steps determine which triads end up major, minor, or diminished.

You do not need to calculate this every time. The pattern is always the same. But understanding why helps you see that chord qualities are not random. They are mathematical consequences of the major scale structure.

Once you know the pattern, finding diatonic chords in any key takes seconds. Write the scale, apply the pattern.

This is why learning the major scale for every key matters. The scale gives you the notes. The pattern gives you the chords. Together they unlock the entire harmonic landscape of the key.

Knowing diatonic chords helps you in multiple ways:

• Learning songs faster: Most songs use mostly diatonic chords. If a song is in G, you know the likely chords before you even listen.
• Writing progressions: You can grab any combination of diatonic chords and they will sound cohesive.
• Understanding substitutions: When a song uses a non-diatonic chord, you notice it. That awareness lets you analyze why it works.
• Connecting scales to chords: Each chord has scale notes that fit over it. Knowing which chord is which degree tells you which notes to emphasize.

On guitar specifically, you can visualize chord shapes at different positions using CAGED. The I chord in G major appears as a G shape at the nut, an E shape at the 3rd fret, a D shape at the 7th fret, and so on.

Do not confuse a chord's scale degree position with its harmonic function. In Level 2, you learned that scale degrees have functional names (tonic, subdominant, dominant). Chords also have functions, but they do not always match their degree number.

This distinction becomes important when analyzing progressions. The ii chord often substitutes for IV because they share the same function, not because 2 and 4 are mathematically related. Function is about harmonic role, not position.

Notice that the vi chord (the minor chord built on the 6th degree) shares all its notes with the major key. In C major, the vi chord is A minor. A minor uses the notes A, C, and E, all from the C major scale.

This vi chord is the relative minor of the key. The same seven notes that create C major diatonic chords also create A minor diatonic chords, just with different functions. This relationship is why songs can shift between relative major and minor so smoothly.