AP Music Theory Unit 5 Essentials: Key Terms & Core Concepts

Key Concept 1

Predominant chords set up the dominant

IV/iv and ii/ii° exist to create momentum toward V. Slotted into the T–PD–D–T model, they explain why some progressions feel purposeful and others feel random. Their seventh-chord forms (ii7 in major, iiø7 in minor) work the same way, with the chordal seventh still resolving down by step.

Predominant Function ii/IV Seventh Chords

Key Concept 2

vi is a tonic substitute; iii is (mostly) avoided

vi shares two notes with I, so it can stand in for tonic — most famously as the destination of a deceptive cadence (V–vi). iii, by contrast, shares tones with both I and V, making its function ambiguous; 18th-century style mostly avoids it in root position, using it occasionally as a passing chord instead.

vi Chord iii Chord Deceptive Cadence

Key Concept 3

Spelling isn't always function — the cadential six-four

A I⁶₄ right before V looks like tonic on paper, but it behaves like a decorated dominant: its upper voices resolve down by step into V, it falls on a strong beat, and it never substitutes for a real tonic arrival. This is the clearest example in the course that Roman-numeral spelling and harmonic function aren't always the same thing.

Cadential 6/4 Function vs. Spelling Dominant

Key Concept 4

Not every six-four chord is cadential

Passing, neighboring, and arpeggiated six-fours all decorate a single underlying harmony rather than creating real harmonic motion. The key to telling them apart is watching the bass: does it move stepwise between two chords (passing), stay put (neighboring), or leap within one chord (arpeggiated)?

Passing 6/4 Neighboring 6/4 Arpeggiated 6/4

T–PD–D–T model

The standard harmonic skeleton of a phrase: tonic, then predominant, then dominant, then tonic again.

Function

Predominant function

The role of IV/iv and ii/ii°: building harmonic momentum that leads into the dominant.

Function

Predominant seventh chord

A seventh chord on a predominant degree: ii7 (minor seventh) in major keys, iiø7 (half-diminished) in minor keys.

Function

vi chord (tonic substitute)

The submediant triad, which shares two notes with I and can stand in for tonic, especially in a deceptive cadence.

vi & iii

iii chord

The mediant triad; rare in root position in this style because it shares tones with both I and V, making its function ambiguous.

vi & iii

Deceptive cadence

V resolving to vi instead of the expected I, using vi's tonic-substitute quality to avoid a full resolution.

vi & iii

Plagal cadence

A IV–I (or iv–I) cadence, moving directly from predominant to tonic without a dominant chord.

Cadences

Phrygian half cadence

A half cadence in minor where iv⁶ moves to V, with the bass falling a half step — named for its Phrygian-mode character.

Cadences

Cadential six-four

A I⁶₄ chord immediately before V at a cadence; spelled like tonic but functions as an embellishment of the dominant.

Six-Four Chords

Six-four resolution

In a cadential 6/4, the 6th and 4th above the bass resolve down by step onto the 5th and 3rd of the following V chord.

Six-Four Chords

Passing six-four

A six-four chord that connects two chords a third apart, with the bass passing stepwise through the intervening scale degree.

Six-Four Chords

Neighboring (pedal) six-four

A six-four chord where the bass stays fixed while upper voices move to neighbor tones and back.

Six-Four Chords

Arpeggiated six-four

A six-four chord formed when the bass simply leaps through the notes of a single harmony.

Six-Four Chords

Metric placement (cadential 6/4)

The rule that a cadential six-four falls on a strong beat, resolving to V on a weaker beat — reinforcing its dominant function.

Six-Four Chords