Music Fundamentals III: Triads & Seventh Chords

What this unit covers

The topics below follow the published Music Theory course framework for Unit 3. Music Theory publishes no per-unit weighting, so there is no percentage to chase here.

Lessons in this unit

• Intervals: Size & Quality14 min · 3 objectivesMeasure an interval’s size by counting letter names inclusively · Assign quality — perfect, major, minor, diminished, or augmented · Invert an interval and predict the resulting size and quality
• Triads & Seventh Chords14 min · 3 objectivesBuild major, minor, diminished, and augmented triads from stacked thirds · Identify a chord’s root, third, and fifth in any inversion · Construct and name the five common seventh-chord qualities

Formulas in Unit 3

Every term in Unit 3

All 18 terms we publish for Music Fundamentals III: Triads & Seventh Chords, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Triad construction

Root, third, fifth. Major has a major third then minor; minor has minor then major; diminished is two minor thirds; augmented is two major thirds.

Roman numeral conventions

Uppercase for major, lowercase for minor, lowercase with ° for diminished, uppercase with + for augmented. The case carries the quality, so getting it wrong is a content error.

Triad inversions and figures

Root position takes no figure or 5/3; first inversion is 6; second inversion is 6/4. The figure counts intervals above the BASS, not above the root.

Seventh chord inversions and figures

Root position 7, first inversion 6/5, second 4/3, third 4/2. Memorize the sequence 7, 65, 43, 42 as a unit.

The five seventh chord qualities

Major-major (IM7), major-minor (V7), minor-minor (ii7), half-diminished (ø7), fully diminished (°7). The name gives the triad quality then the seventh quality.

Half-diminished vs fully diminished

Half-diminished has a diminished triad with a minor seventh, written ø7; fully diminished has a diminished seventh, written °7, and is built entirely of minor thirds.

Why vii°7 needs harmonic minor

The fully diminished seventh chord occurs diatonically only when scale degree 7 is raised, so it belongs to minor keys and to borrowed usage in major.

Doubling in root position

Double the root. Never double the leading tone, and never double the seventh of a seventh chord, because both are tendency tones that must resolve.

Doubling in first inversion

Double the soprano or the root; avoid doubling the bass third if it is the leading tone. In vii°6 double the third, which is the bass.

Doubling in second inversion

Double the bass, which is the fifth of the chord. Six-four chords are unstable and their usage is restricted to four specific types.

The four legal six-four chords

Cadential, passing, pedal (neighbor), and arpeggiated. Any other six-four is a marked error, and the cadential is by far the most tested.

Cadential six-four in context

Notated I6/4–V or V6/4–5/3. It is a dominant with displaced upper voices, not a tonic, so it must fall on a stronger beat than the V it resolves to.

Spelling a chord from a Roman numeral

Build the triad on the scale degree, apply the key signature, then apply any accidental the numeral implies. Skipping the key signature is the most frequent spelling error.

Figured bass realization order

Read the bass, determine the chord and inversion from the figures, then add upper voices observing doubling and range. The figures never change the bass note.

Accidentals in figured bass

A lone accidental applies to the third above the bass; a slashed or plus figure raises that interval. A natural sign cancels rather than lowers.

Chord quality by ear

Major sounds bright and stable, minor darker, diminished tense and unresolved, augmented ambiguous and floating. Practice identifying quality before identifying inversion.

Inversion by ear

Listen to the BASS against the root. Root position is stable, first inversion is lighter and mobile, second inversion sounds suspended and wants to move.

Close vs open spacing

Close spacing keeps soprano, alto and tenor within an octave; open spacing exceeds it. Both are acceptable, but adjacent upper voices must not exceed an octave apart.

What examiners penalize here

• Memorize the two ways the tritone appears: **augmented fourth (fa–ti, e.g. F–B)** and **diminished fifth (ti–fa, e.g. B–F)**. Spotting the tritone between scale degrees 4 and 7 is essential for identifying the dominant chord and voice-leading its resolution.
• Distinguish the two diminished sevenths by the top interval: **half-diminished (ø7)** has a **minor** seventh, **fully diminished (°7)** has a **diminished** seventh (a stack of three minor thirds). In major keys the viiø7 is common; the fully diminished °7 usually signals a minor key or a secondary function.

Practice Music Theory

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Music Theory

1. Unit 1 · Music Fundamentals I: Pitch, Major Scales & Key Signatures, Rhythm, Meter
2. Unit 2 · Music Fundamentals II: Minor Scales & Key Signatures, Melody, Timbre & Texture
3. Unit 3 · Music Fundamentals III: Triads & Seventh Chords
4. Unit 4 · Harmony & Voice Leading I: Chord Function, Cadence & Phrase
5. Unit 5 · Harmony & Voice Leading II: Chord Progressions & Predominant Function
6. Unit 6 · Harmony & Voice Leading III: Embellishments, Motives & Melodic Devices
7. Unit 7 · Harmony & Voice Leading IV: Secondary Function
8. Unit 8 · Modes and Form