What Are Diatonic Chords? (AP Music Theory Must-Know)
In music theory, diatonic chords are chords that naturally occur within a key, using only the notes from that key’s scale — no accidentals or borrowed tones.
Think of them as the “home team” players: every chord is made up of notes that belong to the same musical family (the scale).
- In C major, for example, the notes are C, D, E, F, G, A, B.
Diatonic chords built from C major will only use these notes.
🏛️ Understanding Roman Numerals in Music Theory
To organize and understand these chords, musicians use Roman numerals to represent them. Roman numerals tell two important things:
- The Scale Degree the chord is built on.
- The Quality of the chord (major, minor, diminished, augmented).
Here’s how Roman numeral notation works:
- Uppercase Roman numerals (I, IV, V) = Major chords
- Lowercase Roman numerals (ii, iii, vi) = Minor chords
- Lowercase + ° symbol (vii°) = Diminished chords
🎹 Diatonic Chords in Major Keys
Let’s use C Major as an example:
| Scale Degree | Note | Triad Chord | Roman Numeral |
|---|---|---|---|
| 1st | C | C–E–G | I |
| 2nd | D | D–F–A | ii |
| 3rd | E | E–G–B | iii |
| 4th | F | F–A–C | IV |
| 5th | G | G–B–D | V |
| 6th | A | A–C–E | vi |
| 7th | B | B–D–F | vii° |
✅ Quick Tip:
- I, IV, and V are major chords.
- ii, iii, and vi are minor chords.
- vii° is diminished.
🎵 Diatonic Chords in Minor Keys
In A Minor (the relative minor of C Major), you get slightly different qualities:
| Scale Degree | Note | Triad Chord | Roman Numeral |
|---|---|---|---|
| 1st | A | A–C–E | i |
| 2nd | B | B–D–F | ii° |
| 3rd | C | C–E–G | III |
| 4th | D | D–F–A | iv |
| 5th | E | E–G–B | v |
| 6th | F | F–A–C | VI |
| 7th | G | G–B–D | VII |
But wait!
In harmonic minor, the 7th scale degree is raised (G → G#), so:
- V becomes major (E–G#–B → V)
- vii° becomes diminished (G#–B–D → vii°)
✍️ How Roman Numerals Help with AP Music Theory
Knowing Roman numeral analysis lets you:
- Label chords correctly during the free response section
- Analyze harmonic progressions like a pro
- Compose melodies and harmonizations that make musical sense
- Predict chord functions (like dominant, subdominant, tonic)
🔥 Diatonic Chord Functions in a Nutshell
Every chord has a function in the key:
| Function | Chords | Feeling/Role |
|---|---|---|
| Tonic | I (or i), vi (or VI) | Home base, stability |
| Subdominant | ii (or ii°), IV (or iv) | Prepares for tension |
| Dominant | V, vii° | Tension, needs resolution |
Think of it like a story:
- Tonic = Home
- Subdominant = Journey away
- Dominant = Conflict and suspense
- Tonic (again) = Return home, resolution
📚 Tips to Master Diatonic Chords and Roman Numerals
- Practice labeling real songs and classical pieces.
- Memorize the basic Roman numeral patterns for major and minor keys.
- Understand function (not just the letter names).
- Train your ear to hear progressions (e.g., I–IV–V–I is extremely common).
🎯 Final Thoughts
Mastering diatonic chords and Roman numeral analysis is essential for scoring high on the AP Music Theory Exam — and honestly, it’s the foundation of understanding Western music!
The more you practice recognizing these chords and progressions, the easier composition, analysis, and sight-reading will become.
Stay consistent, trust the patterns, and soon you’ll be thinking in Roman numerals without even trying.
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