AP Music Theory Unit 4 Notes: Function, Cadence, and Phrase in Tonal Harmony

Tonic, Dominant, and Subdominant Functions

In tonal music (the major–minor system used in most of what AP Music Theory calls “common-practice” harmony), chords are not just stacks of notes—they behave like characters in a story. Chord function is the idea that certain chords tend to play certain roles: some sound like “home,” some create tension and point away from home, and some serve as a bridge that leads you toward that tension.

A helpful analogy is gravity. The tonic is the planet (stable center), predominant/subdominant chords are the orbit that starts pulling you away, and dominant chords are the slingshot that creates the strongest pull back toward the planet.

The three primary harmonic functions

Tonic function means stability and arrival. A tonic-function chord sounds at rest, like a point of departure or return. In a phrase, tonic-function harmonies often appear at the beginning (establishing the key) and at cadences (confirming the key).

Dominant function means tension that wants to resolve to tonic. Dominant-function chords contain tendency tones—notes with strong directional pull—that make you expect a particular resolution. This is why dominant-to-tonic motion is the backbone of tonal cadences.

Subdominant function (often called predominant function in AP contexts) means “preparing the dominant.” These chords are less stable than tonic, but they usually don’t have as sharp a need to resolve directly to tonic as dominants do. Instead, they commonly move into dominant harmony, setting up a cadence.

In AP Music Theory, you’ll most often see the functions grouped like this (in a major key):

In minor keys, the same functional categories apply, but one detail matters a lot: dominant function is usually strengthened by raising scale degree 7 (creating a leading tone). That’s why you so often see V (major) and vii° in minor rather than v and VII.

How function is created: tendency tones and scale-degree pull

Function isn’t magic—it comes from how the chord’s scale degrees want to behave.

Dominant function is strongest when the harmony contains:

• Scale degree 7 (the leading tone), which strongly wants to rise to scale degree 1.
• Scale degree 4, which commonly wants to fall to scale degree 3 (especially as part of a V7 chord, where scale degree 4 is the chordal seventh).

In a V7 chord in major (built on scale degree 5), those tendency tones are built in. For example, in C major, V7 is G–B–D–F:

• B (scale degree 7) wants to go to C.
• F (scale degree 4) wants to go to E.

That two-note “pull” is a big reason authentic cadences feel final.

Predominant function often features scale degree 4 and 6 prominently (for example, ii or IV). Those notes don’t “solve” the harmonic story by themselves—they typically set up the dominant.

Tonic function emphasizes scale degree 1 (and often 3 and 5), which do not demand motion the way 7 and 4 do in dominant contexts.

Prolongation: how composers extend a function

A phrase doesn’t usually sit on I for two beats and then immediately cadence. Instead, composers prolong (extend) a function through:

• Neighbor/passing motion in individual voices while the harmony stays tonic.
• Changing inversions (I → I6 → I) to keep motion without leaving tonic function.
• Functional substitutions, like using vi as a tonic-function chord (it shares two tones with I in major).

This matters in AP because you can’t identify function by a single chord in isolation—you have to notice what it’s doing in context. A vi chord might feel like tonic prolongation in one spot, but it might participate in a deceptive cadence in another.

Functional progressions you should recognize

In common-practice harmony, functional motion often follows a “flow”:

• Tonic → Predominant → Dominant → Tonic

Typical examples (Roman numerals):

• I → ii6 → V7 → I
• I → IV → V → I
• I → vi → ii6 → V → I (tonic expansion, then predominant)

Example 1: Function in a simple progression (C major)

Progression: C | Dm | G7 | C

• C = I (tonic)
• Dm = ii (predominant)
• G7 = V7 (dominant)
• C = I (tonic)

Even if you didn’t know the labels, your ear will often hear: stable → moving → strong pull → resolution.

Example 2: Tonic substitution (C major)

Progression: C | Am | Dm | G | C

• C = I (tonic)
• Am = vi (often tonic function here because it shares C and E with I)
• Dm = ii (predominant)
• G = V (dominant)
• C = I (tonic)

A common misconception is thinking “vi is always predominant because it goes before ii sometimes.” In reality, vi is more often categorized as tonic-function in basic functional harmony, unless context clearly points elsewhere.

What goes wrong: common function-identification pitfalls

One of the easiest ways to lose points in analysis is to treat function like a memorization list (“ii is subdominant, V is dominant”) instead of a contextual role.

Watch out for these issues:

• Inversions don’t change function. V6 is still dominant function; ii6 is still predominant.
• Non-chord tones can hide the harmony. A passing tone might make a tonic chord look “weird,” but the underlying function may still be tonic.
• Not every IV chord is “the” subdominant in a cadence. IV can be predominant moving to V, but IV–I can also create a plagal cadence (a different effect).

Exam Focus

• Typical question patterns:
  • Identify the function of a chord (or a group of chords) within a phrase, often tied to a cadence location.
  • Analyze a progression with Roman numerals and label where predominant and dominant function occur.
  • In part-writing/free-response contexts, choose a chord that appropriately prepares a cadence (often predominant → V).
• Common mistakes:
  • Calling vi “predominant” automatically; instead, decide based on whether it behaves like tonic expansion or part of a cadence.
  • Treating inversion symbols (6, 6/5, etc.) as new functions rather than voicing details.
  • Ignoring the raised leading tone in minor when identifying dominant function (v vs V is a big difference in tonal strength).

Cadence Types (Authentic, Half, Plagal, Deceptive)

A cadence is a harmonic punctuation mark: it signals the end of a musical idea (or a resting point inside a larger idea). In AP Music Theory, cadences are central because they connect harmony to phrase structure—many questions essentially ask, “What kind of ending is this, and how strong is it?”

Cadences are identified primarily by:

1. The final two harmonies (especially the last chord)
2. The bass motion (root position vs inversion often changes strength)
3. In authentic cadences, the soprano scale degree at the end

Authentic cadences: the strongest “full stop”

An authentic cadence is a dominant-to-tonic ending: V (or V7) resolving to I (or i in minor). This is the most goal-directed motion in tonal harmony, because dominant function contains strong tendency tones that resolve into tonic stability.

Within authentic cadences, AP commonly distinguishes:

Perfect Authentic Cadence (PAC)

A perfect authentic cadence (PAC) is the strongest version of V–I. In typical AP usage (especially in SATB-style writing and analysis), a PAC occurs when:

• The progression ends V → I (or V7 → I)
• Both chords are in root position
• The soprano ends on scale degree 1 (do/tonic)

Why these details matter: root position reinforces the fundamental bass motion (5 to 1), and scale degree 1 in the soprano makes the sense of arrival unmistakable.

Imperfect Authentic Cadence (IAC)

An imperfect authentic cadence (IAC) is still V–I, but it’s less final than a PAC because one of the “strength” conditions is missing. Common reasons it’s imperfect:

• The soprano ends on scale degree 3 or 5 instead of 1
• One (or both) of the chords is inverted (often V6 → I, or V → I6)

A typical misconception is “IAC means any V–I that isn’t at the end of the piece.” Not true—cadence labels are about harmonic/voicing conditions, not whether the composition continues.

Example: PAC vs IAC (C major)

• PAC example: G (V) → C (I), soprano ends on C, both chords root position.
• IAC example: G7 (V7) → C (I), but soprano ends on E (scale degree 3), or the final tonic is I6.

Half cadence: an “open comma” ending on V

A half cadence (HC) ends on a dominant harmony—most typically V (sometimes V7). The defining feature is that it feels unfinished: it points forward, as if the sentence must continue.

In many common-practice phrases, half cadences appear at the end of an antecedent phrase in a period (you’ll learn this structure in the phrase section). The music “asks a question” by stopping on V, and then the consequent phrase “answers” by reaching tonic.

Important: In AP-style harmonic analysis, a half cadence is named for where it lands, not for where it came from. Many different chords can lead into V:

• I → V (HC)
• ii6 → V (HC)
• IV → V (HC)

If the phrase ends on V (and it sounds cadential), you’re usually in half-cadence territory.

Example: Half cadence (G major)

Progression: G (I) | Am (ii) | D (V)
Ending on D (V) creates a clear “not done yet” feeling.

A common mistake is labeling an ending on V6 as a half cadence without considering style/context. In many AP contexts, half cadences are expected to end on V (often root position). If you see V in inversion at a phrase end, check whether it truly functions as the cadential arrival or whether it’s part of a passing/expanding dominant idea.

Plagal cadence: IV to I (“Amen” sound)

A plagal cadence (PC) is the motion IV → I (or iv → i in minor). It’s generally less forceful than an authentic cadence because IV does not contain the same dominant tension (especially the leading tone) that drives V–I.

Why it matters: plagal motion is a common way composers reinforce tonic after a strong cadence, or create a softer, more reflective close. The nickname “Amen cadence” comes from its frequent use in hymn settings.

Example: Plagal cadence (F major)

Progression: Bb (IV) → F (I)

A frequent misconception is calling any IV–I progression a cadence. It only functions as a cadence when it occurs at a phrase ending with cadential timing (a point of arrival). If IV–I happens mid-phrase as part of harmonic motion, it may not be a cadence.

Deceptive cadence: dominant goes “somewhere else”

A deceptive cadence (DC) happens when you strongly expect V to resolve to I, but instead it resolves to another chord—most commonly V → vi in major (or V → VI in minor).

Why vi works: it shares two notes with I in major (for example, in C major, I = C–E–G and vi = A–C–E). So when V resolves “incorrectly,” the ear still hears a partial sense of arrival—just not the expected one.

Deceptive motion is a powerful phrase-expansion tool. It lets a composer “avoid” a full stop, keep momentum going, and extend the phrase before the real cadence.

Example: Deceptive cadence (C major)

Progression: Dm (ii) | G7 (V7) | Am (vi)
Your ear braces for C major, but gets A minor instead.

A common error is labeling any surprising chord after V as “deceptive.” In AP’s most standard usage, deceptive cadence typically means V to vi (or VI). Other evaded resolutions exist in broader theory study, but for AP exam purposes, V–vi is the core pattern to recognize.

How to identify cadence type step by step (a practical method)

When you’re looking at a score or Roman numerals and need to label the cadence:

1. Find the phrase ending: look for a long note, a rest, a double bar, a breath mark, repeated rhythmic pattern ending, or a clear melodic arrival.
2. Check the last harmony (what chord is sounding at the end):
   • Ends on I (or i): could be authentic or plagal.
   • Ends on V: likely half.
   • Ends on vi/VI after V: likely deceptive.
3. Look at the chord before the last:
   • V to I: authentic (PAC/IAC depending on voicing/soprano).
   • IV to I: plagal.
   • Anything to V: half (as long as V is the cadential goal).
4. For authentic cadences, verify strength:
   • Root position? soprano on 1? If yes, PAC; otherwise IAC.

Exam Focus

• Typical question patterns:
  • Given a short excerpt, identify the cadence at the end of a marked phrase (PAC, IAC, HC, PC, DC).
  • In aural multiple-choice, hear a cadence and choose its type (especially PAC vs HC vs DC).
  • In free-response analysis, label cadences and connect them to phrase structure (antecedent vs consequent).
• Common mistakes:
  • Confusing plagal (IV–I) with authentic (V–I)—check the presence of scale degree 7 in the dominant chord if you’re unsure.
  • Labeling a cadence as PAC without checking the soprano scale degree and root position requirements.
  • Calling any V that appears near the end a half cadence, even if the phrase actually continues and the true cadence arrives later.

Phrase Structure and Period Form

Harmony and cadence are not just “vertical” events—they shape musical time. A phrase is a musical unit that feels like one complete thought, often comparable to a sentence in language. Cadences are the punctuation that tell your ear where that thought ends (or pauses).

In AP Music Theory, phrase structure questions often combine melodic cues (breathing space, contour, repetition) with harmonic cues (cadences, tonic/dominant arrival). If you only look at melody, you can miss how the harmony defines the real endings.

What a phrase is (and how you hear one)

A phrase is typically a span of music that leads to a cadence. Many phrases in common-practice styles are 4 measures long, but phrase length can vary (and AP will sometimes test your ability to see that it’s not always perfectly “4 + 4”).

You can often spot a phrase by a combination of:

• A sense of forward motion toward a goal
• A cadence (harmonic arrival) near the end
• A rhythmic slowdown (longer note values) or a rest
• A melodic arrival on a stable pitch (often scale degrees 1, 3, or 5)

Harmony matters because the same melodic gesture can feel “ending-ish” or “continuing” depending on whether it lands on V (half cadence) or I (authentic/plagal cadence).

Antecedent and consequent: musical question and answer

A very common two-phrase structure is the period, which is built from an antecedent phrase followed by a consequent phrase.

• The antecedent is like a question: it often ends with a weaker cadence, especially a half cadence.
• The consequent is like an answer: it often ends with a stronger cadence, especially an authentic cadence.

The key idea is contrast in closure. The first phrase feels incomplete; the second completes the thought.

Why this matters in harmony

Period structure is one of the clearest places you see functional flow at work:

• Antecedent often builds toward dominant and stops there.
• Consequent often repeats or parallels earlier material but continues past dominant to reach tonic.

So if you’re analyzing phrases and you find an HC in measure 4 and a PAC in measure 8, you should immediately suspect a period.

Period form: how it’s constructed

A period is typically two phrases with a relationship of repetition/parallelism and stronger closure in the second phrase.

Parallel period

A parallel period has similar (often nearly identical) opening material in both antecedent and consequent.

• Measures 1–2: opening idea
• Measures 3–4: leads to HC
• Measures 5–6: opening idea returns (similar to measures 1–2)
• Measures 7–8: continues to PAC/IAC

This is common in melodies where the “question” and “answer” start the same way but end differently.

Contrasting period

A contrasting period still has antecedent → consequent with stronger closure at the end, but the consequent begins differently (less literal repetition).

A misconception to avoid: “If the melody changes, it’s not a period.” Periods can be parallel or contrasting; what makes it a period is the antecedent/consequent relationship and the cadence pattern.

Phrase rhythm details you may encounter

Even within a basic period, composers use techniques that can complicate phrase counting:

• Extension: a phrase is lengthened by extra measures before the cadence (often using predominant → dominant expansion or deceptive motion).
• Elision: the end of one phrase and the start of the next share a measure (the cadence and new beginning overlap).
• Phrase groups: multiple phrases that relate but don’t fit the antecedent–consequent cadence pattern strongly enough to be a period.

AP questions may not always require these terms, but noticing the phenomenon helps you avoid forced “4 + 4” interpretations.

How cadences define phrase structure (worked analysis examples)

Example 1: A textbook period (cadences tell the story)

Imagine this harmonic plan in C major:

• mm. 1–4: I → ii6 → V (ends on V) = half cadence
• mm. 5–8: I → IV → V7 → I (ends on I) = authentic cadence (often PAC if soprano ends on C and both chords are root position)

Even without seeing the melody, the cadence types strongly suggest:

• Phrase 1 (antecedent): ends with HC (question)
• Phrase 2 (consequent): ends with AC (answer)

Example 2: Deceptive cadence as phrase expansion

Suppose a phrase in G major aims for a strong cadence but gets detoured:

• mm. 1–4: I → ii → V
• mm. 5: V → vi (deceptive)
• mm. 6–8: ii6 → V7 → I (authentic cadence)

Here the deceptive cadence prevents closure at the expected moment, extending the phrase. If you were phrase-marking, you would not mark the deceptive arrival as the end of the phrase unless the rhythm/melody clearly stops there too. Most often, DC is a “keep going” move.

How to phrase-mark on the AP exam (a reliable approach)

When asked to identify phrase structure or period form:

1. Locate cadences first. Mark where the harmony clearly arrives (HC, AC, etc.). Those points are strong candidates for phrase endings.
2. Check for grouping into two phrases. Are there two similar-length units, often 4 + 4 or something close?
3. Compare openings. If phrase 2 begins like phrase 1, suspect a parallel period; if not, consider contrasting period.
4. Check cadence strength pattern. Antecedent typically ends weaker (often HC), consequent stronger (often AC). If both phrases end with similar strength, you may be in a phrase group rather than a period.

A classic mistake is to decide “it’s a period” just because it’s 8 measures long. Period is a cadential and thematic relationship, not a measure count.

Exam Focus

• Typical question patterns:
  • Given an excerpt, identify phrase boundaries and label the cadence at each boundary.
  • Determine whether an 8-measure passage is a period, and if so, whether it is parallel or contrasting.
  • In aural contexts, recognize the “question–answer” effect created by HC → AC.
• Common mistakes:
  • Marking phrase endings based only on melodic rests while ignoring that the harmony doesn’t cadence there.
  • Assuming all periods are exactly 4 + 4; instead, let cadences and repetition guide you.
  • Confusing a repeated phrase (phrase group) with a period when both phrases end with equally final cadences (for example, two PACs often suggest repetition rather than antecedent–consequent).