A specific type of phrase structure, the sentence, arose and became prevalent in the Classic/Classical era of Western tonal music (around 1750–1820). Its use continues to this day.

A sentence is a single phrase, which, again, means that it has a single cadence. Not all phrases are sentences, but all sentences are phrases.

A sentence is divided into three subphrases. A subphrase is a unit of music smaller than a phrase; it therefore does not meet all of the requirements of a phrase. Most notably, this means that it does not have to end in a cadence. The three subphrases of a sentence are a basic idea, its immediate repetition (or varied repetition), and a continuation. The two basic ideas are collectively referred to as a presentation. The defining proportions of the subphrases are 1:1:2. In other words, the continuation is twice as long as one of the basic ideas.

Presentation

The presentation often involves some configuration of only tonic- and dominant-function harmonies, as shown in the example progressions below. Furthermore, while not specified, the dominant-function chords in these progressions are typically inverted to avoid creating moments that sound like cadences. Even when the chords are in root position, the lack of predominant-function chords helps prevent the listener from perceiving a cadence.

I-V, I-V (basic idea, basic idea exactly repeated)
I-V, V-I (basic idea, basic idea varied)

When composers vary the basic idea, they commonly transpose it up or down by a step. Traditionally, rhythmic variations or added embellishments are insufficient to consider a basic idea as varied. On the other hand, changes in melodic contour—even when the rhythm is the same—risk transforming the basic idea to the point that the listener will no longer recognize it, therefore nullifying a sentence interpretation.

Continuation

The continuation can be characterized as possessing increased momentum. This is achieved through one or more of the following elements:

Fragmentation
- Fragmentation refers to the segmentation of music into chunks smaller than the basic idea, usually two to five notes.
  - The fragments may be new or taken from the basic idea.
- This is arguably the most important feature of a continuation, though there are some that do not contain it.
An increase in harmonic rhythm
- Harmonic rhythm refers to how often the chords change.
- For example, a presentation might feature one chord per measure, while the continuation contains two chords per measure.
The use of predominant function
Quicker and/or denser rhythms than in the presentation
- Examples of quicker rhythms (shorter durations) are eighth notes as opposed to quarter notes, sixteenth notes as opposed to eighth notes, etc.
- Denser rhythms do not necessarily mean quicker rhythms; instead, the term refers to more notes appearing in a span of time.
  - For instance, imagine that there are sixteenth notes in the presentation, but those notes are separated by rests. Meanwhile, there are only eighth notes in the continuation, but they appear consecutively, with no silence in between.

Phrase Diagram

In order to demonstrate our understanding of a phrase structure, we use a phrase diagram. The image below shows a phrase diagram for a sentence, which includes the following components:

A large phrase arc meaning that there is one phrase.
Above the arc, a label for the phrase structure: a sentence.
Three square brackets below the arc, one for each subphrase (basic idea, basic idea, continuation).
A curly bracket over the first two subphrases for the presentation.
A label for the presentation and for each subphrase: basic idea (b.i.), basic idea (b.i.) , and continuation. The second b.i. may or may not include a prime symbol ( ' ). If used, "basic idea prime" tells us that the second basic idea is varied compared to the first.
A label for the cadence at the end of the phrase arc. The example below shows “PAC," though IAC and HC are also acceptable cadences for sentences.
Measure numbers below the brackets that indicate how long each subphrase is. If measure numbers are not provided for the music you are diagramming, you can use numbers beginning on 1. If measure numbers are provided, use the actual measure numbers.