Periods and Sentences

The previous lesson described a four-measure phrase as the equivalent of a sentence in prose.

One sentence does not make a paragraph. Only when two of them answer one another does a paragraph-sized unit appear. The smallest such unit is the period.

Period — question and answer

A period is eight measures made from two four-measure phrases. Simply placing them side by side is not enough; there are conditions.

• the first half, the antecedent, stops on a half cadence
• the second half, the consequent, closes with an authentic cadence

Stopping the first half on the dominant tells the listener that nothing has finished. The second half then arrives and lands on the tonic, supplying the answer. That is why the pair is described as question and answer.

Reversed, it does not work. Closing first and then hanging in the air breaks the unit into two fragments.

The right order, and the reverse

▶Half cadence → authentic cadence▶Authentic cadence → half cadence

Check: The first divides on a half cadence and then closes with an authentic cadence. The eight measures should register as one unit. The second swaps the order, finishing completely at measure four and then stopping mid-air. Identical chords; the order alone destroys the coherence.

the answer follows the question

Parallel and contrasting periods

Periods divide into two kinds according to how the second half begins.

A parallel period opens its consequent with the same motive as the antecedent — walking the same road partway and changing only the ending. It is the most common type and the easiest for a listener to follow.

A contrasting period opens the consequent with different material. The change of material provides more variety, but the sense of unity is weaker than in a parallel period.

What matters is that the cadential scheme is identical in both. Half cadence to authentic cadence is the shared skeleton, and only the melodic opening differs. So these are not two different forms but two ways of writing the same period.

Parallel period beside contrasting period

▶Parallel period▶Contrasting period

Check: Both are eight measures, divided the same way from half cadence to authentic cadence. In the first, the opening shape returns at measure five, so the pairing is immediately obvious. In the second, measure five begins new motion — fresher, but with a weaker sense of connection. Note also that listening to the harmony alone would not tell them apart.

the second half begins with the same shape

Check

An eight-measure theme begins measure five with exactly the motive of measure one, stops on V in measure four, and lands on I in measure eight. What is it?

A sentenceA parallel periodBinary formA contrasting period

Check answer

Sentence — state briefly, develop at length

There is a second design for an eight-measure unit, built on completely different principles: the sentence.

It is assembled as 2+2+4.

Part	Length	Role
Basic idea	2 measures	states the material
Repetition	2 measures	restates it, usually at a new pitch level
Continuation and cadence	4 measures	fragments and accelerates, closing with a cadence

The first four measures form no cadence. There is only the basic idea and its repetition, with the harmony staying around the tonic. Dividing here would stop the momentum.

Then the second four measures move decisively. The motive is chopped into smaller pieces, the harmonic rhythm speeds up, and an authentic cadence lands at the end. A sentence accelerates into its ending.

Period and sentence back to back

▶Period (4+4, balanced)▶Sentence (2+2+4, accelerating)

Check: Both are eight measures. The first divides clearly at measure four and the two halves balance each other. The second never divides and accelerates continuously toward its ending. Same length, and the difference between a center of gravity in the middle and one at the back produces two entirely different experiences.

half cadence in the middle

Choosing between the two designs

Neither design is superior. The choice is about where to put the center of gravity in time.

	Period	Sentence
Division	4+4	2+2+4
Midpoint	clearly divided by a half cadence	not divided
Center of gravity	the middle (symmetrical)	the second half (accelerating)
Suits	settled themes, song	development sections, building passages

Music with lyrics leans toward periods, because two lines of verse correspond naturally to two phrases. Instrumental development sections lean toward sentences, because they can push on without dividing.

Check

An eight-measure theme restates its opening two-measure shape at measure three, then moves in finer values from measure five and lands on I at measure eight. There is no half cadence anywhere. What is it?

Ternary formA parallel periodA sentenceA contrasting period

Check answer

Hybrids — the two designs combine

Real music frequently produces eight-measure units that mix elements of both. The most common hybrid takes a period’s antecedent as its first half and a sentence’s continuation as its second.

First 4 measures: antecedent (divided by a half cadence)
Second 4 measures: continuation and cadence (fragmenting, accelerating, authentic cadence)

Divide clearly in the first half, then accelerate to a close in the second. It takes both the legibility of symmetry and the drive of acceleration.

The reverse combination also exists: a sentence’s presentation (basic idea and repetition) in the first half, closed by a well-ordered consequent in the second.

The existence of hybrids means that period and sentence are not mutually exclusive categories. They are two poles of one design question — how to allocate eight measures — with a continuous range between them. When an analysis is ambiguous, it is closer to the reality to ask separately whether the first half divides and whether the second half accelerates than to force a single label.

Expansion and compression

Real music also produces periods that are not exactly eight measures.

Expansion adds measures by deferring the cadence. Inserting a deceptive cadence just before the close allows the passage to be stretched by any number of measures. This is the use Unit 4 had in mind in saying that deceptive cadences are for when you do not want the piece to end.

Compression does the opposite, shortening the second half to close in a hurry. It appears in passages of urgency.

Both only work because eight measures was expected. Knowing the standard shape is what allows a departure from it to communicate.

▶ Compare period and sentence in the form viewer Select the parallel period, the contrasting period, and the sentence in turn and play just the first half of each. The display makes it visible that only the sentence produces no cadence there Form Viewer → ▶ Practice identifying divisions in the cadence trainer Analyzing periods presupposes being able to say instantly whether a cadence was a half or an authentic one Cadence Trainer →

Periods and sentences are designs inside the small container of eight measures. The next lesson enlarges the container and takes up the shapes made when several periods combine.