Diatonic Harmony & the Circle of Fifths

Music Theory Fundamentals

Chapter 7 · Diatonic Harmony & the Circle of Fifths

Every tool this chapter needs already exists from earlier in this course — Chapter 3's own major scale and Chapter 5's own triad-construction rules combine directly to produce a real, fixed, predictable pattern that governs harmony in every major key.

Building a Triad on Every Scale Degree

Stack a third and another third on top of each note of the major scale, using only notes already in that scale, and a genuinely fixed pattern of triad qualities falls out automatically — not by convention, but as a direct mechanical consequence of the scale's own real interval structure.

Deriving it directly from C major

I (C-E-G): major 3rd + minor 3rd → Major
ii (D-F-A): minor 3rd + major 3rd → Minor
iii (E-G-B): minor 3rd + major 3rd → Minor
IV (F-A-C): major 3rd + minor 3rd → Major
V (G-B-D): major 3rd + minor 3rd → Major
vi (A-C-E): minor 3rd + major 3rd → Minor
vii° (B-D-F): minor 3rd + minor 3rd → Diminished
This pattern — major, minor, minor, major, major, minor, diminished — holds in every major key, not just C, since it follows directly from the major scale's own fixed W-W-H-W-W-W-H pattern (Chapter 3).

Roman Numeral Analysis

Music theory names these seven diatonic chords with Roman numerals matching their scale degree, using a real, standard convention: uppercase for major chords, lowercase for minor chords, and a small ° symbol for the diminished chord. This is exactly why the pattern above is written I-ii-iii-IV-V-vi-vii° — the case of each numeral already tells you the chord's own quality before you even look at the notes.

Scale Degree Names

Each scale degree also carries a real, traditional name, most of them meaningfully describing the degree's own position relative to the tonic:

Degree	Name	Real meaning	Roman numeral (major key)
1	Tonic	The tonal center — note of final resolution	I
2	Supertonic	"Above the tonic" — one whole step up	ii
3	Mediant	Midway between tonic and dominant	iii
4	Subdominant	The "lower dominant" — a fifth below the tonic	IV
5	Dominant	Second in importance to the tonic	V
6	Submediant	The "lower mediant," mirroring the mediant below the tonic	vi
7	Leading Tone	A half-step below the tonic, pulling melodically toward it	vii°

The Circle of Fifths and Key Relationships

Chapter 3's own circle of fifths does more than generate key signatures — it also predicts which keys sound most closely related to each other. Neighboring keys on the circle (like C major and G major, one real perfect fifth apart) share six of their seven scale tones, differing by only one real accidental — exactly why modulating to a neighboring key on the circle of fifths feels so smooth, while modulating to a distant key feels far more abrupt.

Hands-On Exercises

Exercise 1

Using the real G major scale (G-A-B-C-D-E-F♯-G) from Chapter 3's own material, build the triad on the 5th scale degree (V) and identify its quality using semitone counts, per this chapter's own method.

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Exercise 2

Using this chapter's own scale-degree-name table, name the scale degree traditionally called the "subdominant," and state which Roman numeral and triad quality it takes in a major key.

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Exercise 3

Explain, using this chapter's own material and Chapter 3's circle of fifths, why C major and G major share six of their seven scale tones, and name the one note that differs between them.

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Chapter 7 Quick Reference

Building a triad on every degree of the major scale produces a fixed real pattern: I-ii-iii-IV-V-vi-vii° (major, minor, minor, major, major, minor, diminished)
Roman numeral analysis — uppercase = major, lowercase = minor, ° = diminished
Real scale-degree names: tonic, supertonic, mediant, subdominant, dominant, submediant, leading tone
Neighboring keys on the circle of fifths (Ch.3) share 6 of 7 diatonic chords, differing by one accidental
Next chapter: Chord Progressions & Cadences