Diatonic chords are the triads and seventh chords built on each scale degree of a key, using only notes from that key. Roman numerals label each chord by its scale degree and quality: uppercase for major, lowercase for minor, with a degree sign for diminished and a plus sign for augmented.
Why This Matters for the AP Music Theory Exam
This topic builds the labeling system you use for the rest of AP Music Theory. Once you can build a chord on any scale degree and name its quality, you can write Roman numerals for what you hear and see. That skill carries into harmonic dictation, where you identify chords inside a progression instead of in isolation, and into score analysis, where you mark out how a passage moves from chord to chord.
Roman and Arabic numeral notation shows up across the exam, including in free-response questions that ask you to use this system. Getting fluent now means you spend less time decoding symbols later and more time hearing and writing how harmonies function.
Key Takeaways
- Diatonic chords are built only from the notes of a given key's scale, stacked in thirds.
- Roman numerals show both the scale degree of the root and the chord quality: uppercase = major, lowercase = minor, lowercase with ° = diminished, uppercase with + = augmented.
- In a major key the qualities are I, ii, iii, IV, V, vi, vii°, and this pattern is the same in every major key.
- In natural minor the pattern is i, ii°, III, iv, v, VI, VII; raising the leading tone changes v to V and VII to vii°.
- Each scale degree has a name (tonic, supertonic, mediant, subdominant, dominant, submediant, leading tone), and chords can be called by those names too.
- The root is the note a chord is built on in root position and stays the same even when the chord is inverted.
What Diatonic Means
Diatonic means within the key, using only the notes of that key's scale. If you are in C major, the diatonic notes are C, D, E, F, G, A, and B, so every diatonic chord uses only those pitches. Triads and seventh chords built on each scale degree make up the diatonic chords of that key, and each one can turn out major, minor, diminished, or augmented depending on the intervals that stack up.
Diatonic Chords in Major Keys
A major scale gives you seven diatonic triads, one on each scale degree, labeled with Roman numerals. To build them, write the scale, then stack a third and a fifth above each note using the lines and spaces of the staff.
To work through an example, start with the C major scale:
Then build a triad on each scale degree by stacking thirds:
Next, check the quality of each chord by looking at the intervals inside it (major, minor, diminished, or augmented):
The useful part is that the quality pattern you find in one major scale applies to every major scale. Here is how the chords line up:
- I on scale degree 1 is major
- ii on scale degree 2 is minor
- iii on scale degree 3 is minor
- IV on scale degree 4 is major
- V on scale degree 5 is major
- vi on scale degree 6 is minor
- vii° on scale degree 7 is diminished
Notice how the Roman numeral itself tells you the quality. Uppercase means major, lowercase means minor, and the lowercase vii with a ° means diminished.
Roman Numerals and the Root
Roman numerals identify a chord by its root, the note it is built on in root position. The root stays the same even if the chord is rearranged. In C major, G-B-D is a V chord, and B-D-G is still a V chord because the root is still G. You will learn how figured bass shows inversions in Unit 3.3.
You can also name chords by their scale degree names. The V chord is the dominant, and a seventh chord built on it is the dominant seventh, a very common chord. The ii chord is the supertonic. These names stay consistent and work in both major and minor.
Diatonic Chords in Minor Keys
Minor keys follow a different quality pattern. Using natural minor, the chords come out like this:
- i on scale degree 1 is minor
- ii° on scale degree 2 is diminished
- III on scale degree 3 is major
- iv on scale degree 4 is minor
- v on scale degree 5 is minor
- VI on scale degree 6 is major
- VII on scale degree 7 is major
The seventh scale degree is the tricky one. With natural minor and no raised seventh, the chord on scale degree 7 is major (VII). If you raise the seventh (harmonic minor), that chord becomes diminished (vii°). Composers use both depending on the context.
Let's build the chords in G minor. Start with the scale:
Then stack triads on each scale degree:
For the V chord, the third is raised a half step so the chord becomes major. In natural minor the chord on scale degree 5 is minor, but raising the leading tone (the seventh scale degree) turns it into a major V.
Now identify the quality of each chord by looking at its triad:
The same raised seventh that makes V major also turns VII into vii°. That is why the chord on scale degree 7 in minor can be either major or diminished, depending on which form of minor the composer uses.
One more thing to remember: there are no naturally occurring augmented chords in a diatonic scale. An augmented chord has to be altered from a major chord.
Here is a summary of the scale degrees, their names, and their Roman numerals in both major and minor:
How to Use This on the AP Music Theory Exam
Notation and Analysis
When you label chords with Roman numerals, do three things every time: find the root, name the scale degree it sits on, and check the quality. Match the case of the Roman numeral to the quality, and add ° or + when needed. This is the system used to mark harmonies in a score.
Aural Recognition
Practice hearing chords inside a progression, not just in isolation. The exam expects you to recognize how chords relate to each other in a key, so train your ear on common moves like tonic to dominant. Comparing major and minor versions of the same chord helps you hear quality differences faster.
Quick Self-Check
Can you spell all the diatonic triads in B major? Can you spell a dominant chord in A major? If you can build any triad on any scale degree and name its quality, you are ready for the inversion and figured bass material that comes next.
Common Misconceptions
- The root and the bass note are not always the same. The root is the note a chord is built on in root position; the bass is whatever note is lowest. An inverted chord keeps the same root even though the bass changes.
- Uppercase versus lowercase is not random. The case of a Roman numeral tells you the quality: uppercase for major, lowercase for minor.
- The chord on scale degree 7 in minor is not fixed. It is major (VII) in natural minor and diminished (vii°) when the leading tone is raised.
- Augmented triads are not diatonic. They do not occur naturally in a major or minor scale and must be created by altering a major chord.
- A dominant seventh is not only a major-key chord. Because the V chord is made major in minor by raising the leading tone, the dominant seventh appears in both major and minor.
Related AP Music Theory Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
Arabic numerals | Numbers (1, 2, 3, etc.) used in figured bass notation to denote specific intervals above a given bass note. |
augmented triad | A three-note chord indicated by an uppercase Roman numeral with a plus sign (+), consisting of a root, major third, and augmented fifth. |
bass line | The lowest melodic line in a musical composition that often implies harmonic progressions through its note choices. |
chord quality | The classification of a chord based on the specific intervals between its pitches, such as major, minor, diminished, or augmented. |
diatonic chords | Chords built on the scale degrees of a given key using only the notes of that key. |
diminished triad | A three-note chord built on a root with a minor third and a diminished fifth. |
dominant seventh chord | A seventh chord built on the fifth scale degree of a key, typically used to create harmonic tension that resolves to the tonic. |
major triad | A three-note chord built on a root with a major third and a perfect fifth. |
mediant | The third scale degree, located in the middle between the tonic and dominant. |
minor triad | A three-note chord built on a root with a minor third and a perfect fifth. |
Roman numeral analysis | A system of notation using Roman numerals to identify chords and their harmonic function within a key. |
root | The fundamental note of a chord upon which the chord is built. |
scale degree | The position of a pitch within a scale, identified by name or number relative to the tonic. |
seventh chords | Chords built on a triad by adding a note a seventh above the root, creating four-note harmonies with specific qualities. |
subdominant | The fourth scale degree and its associated chord (IV or iv), which functions as a predominant harmony leading toward the dominant or tonic. |
supertonic | The second scale degree, located one step above the tonic. |
tonic | The first scale degree and the primary harmonic center of a key, providing the sense of resolution and stability. |
triads | Three-note chords consisting of a root, third, and fifth. |
Frequently Asked Questions
What are diatonic chords?
Diatonic chords are triads and seventh chords built from the notes of one key. If a chord uses only pitches from the key, it is diatonic to that key.
How do Roman numerals work in music theory?
Roman numerals label the scale degree of a chord's root and show chord quality. Uppercase means major, lowercase means minor, lowercase with a degree sign means diminished, and uppercase with a plus sign means augmented.
What is the diatonic triad pattern in major keys?
In major keys, the diatonic triad pattern is I, ii, iii, IV, V, vi, vii°. The pattern is the same in every major key because the major scale interval pattern stays the same.
What is the diatonic triad pattern in minor keys?
In natural minor, the pattern is i, ii°, III, iv, v, VI, VII. When the leading tone is raised, v often becomes V and VII can become vii°.
What is a I-IV-V chord progression?
A I-IV-V progression uses chords built on scale degrees 1, 4, and 5. In a major key, all three are major triads and they create a common tonic, subdominant, dominant framework.
What is the difference between root and bass note?
The root is the pitch a chord is built on in root position. The bass note is the lowest sounding pitch. In inversions, the bass changes, but the root and Roman numeral stay tied to the chord identity.