Volume III · Chapter 25
Phrase Structure
MUSIC 21A · C-ID MUS 140Chapter quiz →
In the musicianship bookChapter 16: Phrases in Larger Shapes
Introduction: How Music Breathes
Harmony tells us where a passage is going; phrase structure tells us how it gets there. In tonal music, ideas group into units of predictable size (usually two, four, and eight bars) that end with cadences, much as words group into clauses and sentences that end with punctuation. Recognizing these units lets you hear a piece the way its composer built it, and gives you templates for your own writing.
- A motive is the smallest recognizable musical idea: a few notes with a distinctive rhythm and contour.
- A basic idea (usually two bars) is the opening gesture of a theme, often built from one or two motives.
- A phrase is a complete musical thought that ends with a cadence (usually four bars).
- Phrases combine into larger themes: sentences, periods, and their relatives.
Click any example to hear it.
Motives and Their Development
Composers rarely state an idea once and drop it. They repeat it, move it, turn it upside down, stretch it, and break it apart. These are the standard techniques of motivic development:
| Technique | What changes |
|---|---|
| Repetition | Nothing: the motive is stated again exactly. |
| Sequence | The motive is restated starting on a different pitch (usually a step or third higher or lower). |
| Inversion | Each interval changes direction: up becomes down. |
| Retrograde | The notes come in reverse order (rare in tonal themes). |
| Augmentation / diminution | The rhythm is stretched (longer values) or compressed (shorter values). |
| Fragmentation | Only part of the motive is used, often repeated in quick succession. |
| Intervallic change | The rhythm and contour stay, but one or more intervals are widened or narrowed. |
The Sentence
A sentence is an eight-bar theme in two halves:
- Presentation (bars 1–4): a two-bar basic idea, then its repetition, often transposed so that it sits over dominant harmony (I → V).
- Continuation (bars 5–8): the music speeds up. Typical signs are fragmentation into one-bar or half-bar units, faster harmonic rhythm, and a sequence, leading to a cadence (PAC, IAC, or HC).
The Period
A period is a pair of phrases in a question-and-answer relationship:
- The antecedent (bars 1–4) ends with a weak cadence, usually an HC (sometimes an IAC).
- The consequent (bars 5–8) ends with a stronger cadence, usually a PAC.
- In a parallel period the consequent begins like the antecedent (at least the basic idea). In a contrasting period it begins with different material.
If both phrases end with the same strength of cadence (two PACs, say) the result is not a period, just a phrase group or a repeated phrase.
The Double Period
Scale the period up and you get a double period: sixteen bars in which the first eight-bar half acts as one big antecedent (ending with an HC or IAC) and the second half as one big consequent (ending with a PAC). Each half usually contains two phrases. If the second half starts like the first, it's a parallel double period; otherwise, a contrasting double period.
Hybrid Themes
Many themes mix the parts of the sentence and the period. Name each half by its function:
| First half (bars 1–4) | Second half (bars 5–8) | Name |
|---|---|---|
| Antecedent (b.i. + contrasting idea, HC) | Continuation (fragmentation, cadence) | Hybrid: antecedent + continuation |
| Compound basic idea (b.i. + contrasting idea, no cadence) | Consequent (b.i. returns, PAC) | Hybrid: compound basic idea + consequent |
| Compound basic idea | Continuation | Hybrid: compound basic idea + continuation |
| Presentation (b.i. + repetition) | Consequent-like cadence phrase | Sentence (the standard model) |
Don't worry about forcing every theme into a box: ask what each half does (presents, continues, closes) and where the cadences are.
Phrase Expansion
Real phrases are often longer than four bars. Composers expand them in three main places:
- Introduction: a bar or two of accompaniment before the theme starts.
- Internal expansion: material inside the phrase is repeated or stretched. A favorite is the evaded (deceptive) cadence: the expected V–I becomes V–vi, and the cadence must be tried again.
- Cadential extension: after the PAC, the cadence is repeated or prolonged (a tag, a codetta, or repeated I–V–I or IV–I).
Elision
When the last bar of one phrase is also the first bar of the next, the phrases are elided (overlapped). The cadence arrives and the new phrase starts on the same downbeat, so the music never stops to breathe. Elisions are common at the end of a theme, where they push the piece forward into the next section, and they make phrase counts look odd (4 + 4 = 7 bars).
Drawing Phrase Diagrams
- Find the cadences first and write each one (HC, IAC, PAC, DC) under its bar.
- Draw an arc over each phrase (cadence to cadence).
- Label phrases with lowercase letters: the same letter for the same material, a prime (a′) for a varied return, a new letter for new material.
- Group phrases with larger arcs and name the theme: sentence, parallel or contrasting period, double period, phrase group, or hybrid.
- Note any expansion, evaded cadence, or elision.
Try It: Period Builder
Period Builder
Build an eight-bar theme in G major from two four-bar phrases. Choose how the first phrase ends, how the second begins, and how it ends; then see (and hear) what kind of theme you've made.
Summary
- A motive develops by repetition, sequence, inversion, augmentation/diminution, fragmentation, and intervallic change.
- A sentence = presentation (b.i. + repetition) + continuation (fragmentation → cadence).
- A period = antecedent (weak cadence) + consequent (stronger cadence); parallel or contrasting. A double period does the same with eight-bar halves.
- Hybrid themes combine sentence and period functions.
- Phrases expand by introductions, internal expansion (e.g., the evaded cadence), and cadential extension; elision overlaps phrases by a bar.
- Phrase diagrams show cadences, phrase arcs, letters for material, and the overall theme type.