A sentence is type of theme. Like the period, the prototypical sentence is eight measures long and contains two four-measure phrases. The first of these is called the presentation phrase and the second is the continuation phrase.

Presentation phrase (mm. 1–4)

The presentation phrase begins the sentence and has two primary components, one melodic and the other harmonic. Melodically, it contains two repeated basic ideas (BI). Harmonically, it prolongs the tonic, by means of either a subsidiary harmonic progression or contrapuntal chords. (See the Harmonic syntax resource for information on harmonic prolongation.)

Mozart, K. 283, i., mm. 1–4:

Here, the basic idea begins with the pickup to measure 1 and ends with the F-sharp on the downbeat of m. 2. It’s then repeated. Notice that a basic idea does not need to be repeated exactly. Rather, a basic idea is characterized by its melodic and motivic components, which usually involve rhythm and melodic contour.

Harmonically, these four measures prolong tonic. Measures two and three are contrapuntal chords that are surrounded by the tonic:

I V4/3 V6/5 I
or
T(1 D(2 7)n 1)

Altogether, the two expressions of the basic idea and the tonic prolongation in this phrase exhibit presentation function. Hence, the name “presentation phrase.”

Continuation phrase (mm. 5–10)

Continuation phrases acquire momentum that leads to the cadence that ends the sentence. Notice in the diagram above that three types of cadence can end a sentence: PAC, IAC, or HC.

Continuation phrases begin with continuation function. We usually characterize continuation function as having one or more (but not necessarily all) of the following five characteristics, all of which create this momentum toward the cadence. (See Classical theme functions for more detail.)

• Fragmentation
• Liquidation
• Sequential repetition
• Accelerated Melodic Rhythm
• Acceleration of harmonic rhythm

Mozart, K. 283, i., mm. 1–10:

In this example, the dotted rhythm that formed one half of the basic idea in mm. 1–4 is fragmented beginning with the pickup to m. 5: the two-measure size established by the basic ideas in the presentation is cut into smaller fragments—here, one-measure long fragments.

This example also features acceleration of melodic rhythm (the sixteenth notes in mm. 7–8) and acceleration of harmonic rhythm: while in mm. 1–6 the harmony changed once per measure, beginning at m. 7 the harmonic rhythm speeds up, changing twice per measure.

Mm. 5–8 exhibit continuation function, but mm. 9–10 exhibit cadential function. While mm. 5–8 continued the tonic prolongation by means of contrapuntal chords, mm. 8–10 employ a typical cadential progression:

I IV6 Cad.6/4 V7 I
or
T1 S6 D5 — T1

And while mm. 5–8 employed fragments of the basic idea (the “characteristic” elements of the melody), mm. 8–10 employ elements unrelated to the basic idea (the “conventional” elements—scales and arpeggios, ending with a descending contour).

In a sentence, the continuation phrase always contains both continuation and cadential functions.

Phrase length

Note the length of the phrases in this example: four-bar presentation phrase followed by six-bar continuation phrase. The prototypical phrase is four measures, but this is commonly altered by Classical composers. However, as this theme exemplifies, phrases are more often expanded to something longer than four measures than compressed to something less than four measures. Also, as exemplified by this theme, continuation phrases of non-standard lengths are more common than presentation phrases of non-standard lengths.