The chords that belong to a key

The key gives you a useful family of chords. It is a starting point for listening and practice, not a rule that every melody note needs a new chord.

The C major family

Use the C major notes C–D–E–F–G–A–B. Build each chord by taking a note, skipping a scale note, taking the next, skipping again and taking the next. Continue into the next octave when needed.

Uppercase numerals mean major chords, lowercase mean minor, and ° means diminished. The same pattern works in another major key: in G major, I–IV–V means G–C–D.

Begin with three chords

Practise C → F → G → C (I–IV–V–I), four counts per chord. Then try C → Am → F → G (I–vi–IV–V). These are exercises, not automatic accompaniments for every hymn. Keep common notes and use nearby inversions.

Choose changes that support the melody

Listen at strong beats and phrase endings. A sustained E might belong to C, Am or Em; the surrounding melody and bass help you decide. Passing notes need not trigger chord changes. Follow a reviewed arrangement when you have one, and agree on changes with the other players.

Relative minor is related, but has a different home

C major and A natural minor share the same key signature. A minor makes A its home. A natural minor gives Am–Bdim–C–Dm–Em–F–G; in many minor-key passages E major or E7 replaces Em, using G♯ to lead back to A. Do not assume a minor song stays on white notes.

Further reading: Chords in major and Chords in minor — Music Theory for the 21st-Century Classroom.