Diatonic chords: build the chords in any key
Diatonic harmony is a map of the chords built from one scale. It is not a rule that forbids outside chords; it is a practical reference for hearing function, checking a chord sheet and understanding why many progressions feel connected.
Build the major-key set
Number the notes of a major scale I through VII and stack thirds. The chord qualities follow a stable pattern: major, minor, minor, major, major, minor, diminished. In C major that gives C, Dm, Em, F, G, Am and B°.
Hear scale-degree function
I usually feels like home. IV moves away from home, while V creates a strong pull back to I. vi is the relative minor and ii and iii often connect the main functions. Learning the sound of these roles transfers to every key.
Use the map to review a chord sheet
- Find the likely key.
- Write its seven diatonic chords.
- Compare the chart with that set.
- If an outside chord appears, listen before changing it; it may be a secondary dominant, modal borrowing or modulation.
Minor keys need one extra idea
Minor-key harmony often raises the seventh scale degree to create a stronger dominant. That is why A minor music may use both Em and E or E7.
Practice
Take three songs in different keys and label I, IV, V and vi. Play the same functional pattern in each key. The goal is to recognize relationships rather than memorize isolated chord names.