Major keys and diatonic chords

Songs in a key use mostly the chords that can be built from that key's scale. Once you know the seven diatonic chords and their Roman numerals, you can predict which chords a song will use, and move it to any key.

By the end of this lesson you will be able to:

• Build the seven diatonic triads of a major key.
• Name chords with Roman numerals (I, ii, iii, IV, V, vi, vii°).
• Recognise I, IV and V as the primary chords of a key.

A key is a home note plus the seven notes of its scale. Diatonic means "from the key". To find a key's chords, take each note of the scale and stack thirds above it using only notes of the scale.

In G major (G A B C D E F♯):

• on G: G B D = G major
• on A: A C E = A minor
• on B: B D F♯ = B minor
• on C: C E G = C major
• on D: D F♯ A = D major
• on E: E G B = E minor
• on F♯: F♯ A C = F♯ diminished

The pattern of qualities is the same in every major key: major, minor, minor, major, major, minor, diminished.

Roman numerals name the chord by its scale degree: I ii iii IV V vi vii°. Upper case is major, lower case is minor, and ° is diminished. Because numerals describe the chord's role rather than its name, they work in every key.

The three major chords I, IV and V are the primary chords. A huge amount of folk, rock and country uses only those three.

Other keys, same pattern:

• C major: C, Dm, Em, F, G, Am, B°
• D major: D, Em, F♯m, G, A, Bm, C♯°
• A major: A, Bm, C♯m, D, E, F♯m, G♯°

Finding the chords of a key you do not know. Say you want the chords of D major.

1. Write the scale: D E F♯ G A B C♯.
2. Build the triads: D F♯ A (major), E G B (minor), F♯ A C♯ (minor), G B D (major), A C♯ E (major), B D F♯ (minor), C♯ E G (diminished).
3. Apply the pattern to check: major, minor, minor, major, major, minor, diminished. Yes.

So D, Em, F♯m, G, A and Bm all belong to D major. A song in D that uses D, G and A is playing I, IV and V.

Do these in order. Each one is small. Tick the ones you have practised.

• 1.Build the chords of G major

  4 min

  1. Write the G major scale: G A B C D E F♯.
  2. On each note build a triad using only notes of the scale: skip one, take one, skip one, take one.
  3. Label each triad as major, minor or diminished, and give it a Roman numeral.

  You have it when

  • You get G, Am, Bm, C, D, Em, F♯°, and numerals I, ii, iii, IV, V, vi, vii°.
  I practised this one

• 2.Play the chords in order

  4 min · 50 → 70 BPM

  1. Play G, Am, Bm, C, D, Em in that order, four beats each.
  2. Use the A-shape barre for Bm.
  3. Say the Roman numeral of each chord as you play it.

  You have it when

  • You can play all six chords in order and say their numerals without stopping.
  I practised this one

• 3.I–IV–V in three keys

  4 min · 60 → 90 BPM

  1. Play I, IV and V in G: G, C, D.
  2. Play them in C: C, F, G.
  3. Play them in D: D, G, A.
  4. Four beats each; repeat the progression twice in each key.

  You have it when

  • You can play I–IV–V in all three keys without looking at the chart.
  I practised this one

Play each chord for four beats at 50 BPM, then raise the tempo. The focus is on understanding, not speed.

• Forgetting that the quality of each chord is fixed by the key. In any major key the pattern is the same: major, minor, minor, major, major, minor, diminished.
• Writing numerals without checking case. Upper case for major (IV), lower case for minor (vi), and ° for diminished.
• Ignoring the diminished chord because it is rare. You do not need to play it yet. Just know it exists and where it sits.

Brown Eyed Girl is I–IV–I–V in G, and Knockin' on Heaven's Door uses I, V, ii and IV. Once you can name the numerals, you can see why those songs sound related.

Try it in: Brown Eyed Girl (chord version), Knockin' on Heaven's Door.

Short version (10 minutes)

Write out the seven chords of C major with numerals, then play I, IV and V.

Standard version (about 30 minutes)

Do all three exercises, then work out the diatonic chords of D major on your own and check against the lesson.

• Diatonic chords are built from the notes of the key.
• Pattern: I ii iii IV V vi vii°.
• Upper case for major, lower case for minor.
• I, IV and V are the primary chords.

You are ready to move on when you can honestly say:

• I can build the diatonic triads of G, C and D major.
• I can name chords with Roman numerals.
• I can play I, IV and V in three different keys.