Harmonic syntax concerns the norms or principles according to which harmonies (chords) are placed into meaningful successions. These norms include progressions that are more or less common than others. Those norms generate expectations for listeners familiar with the style: if IV–V is more common than IV–VI, the appearance of a IV chord generates an expectation that the next chord is more likely to be V than it is to be VI.

In Western classical music, harmonies generally group into three harmonic functions—tonic (T), subdominant (S), and dominant (D)—and these functions group together chords that progress to and from other chords in similar ways.

On a local level (chord-to-chord progressions), we can summarize the tendencies of these functions with the cycle T–S–D–T. That is, harmonies tend to progress through a cyclical progression of those three functions:

T → S → D → T → and so on . . .

That does not rule out T progressing to D, D progressing to S, etc. But it does mean that those progressions are less common.

Higher-level musical structures also impact the norms according to which these harmonic functions progress. For now, we will consider one higher-level structure that influences chord-progression tendencies: the phrase. For now, we will limit our study to isolated, complete, self-sufficient phrases. This is an idealized, oversimplified setting—like strict voice-leading—which is useful for learning the basics. Some such phrases even exist in real music! But most of the time there are a number of competing factors that influence the chord-progression strategies employed by a composer at any given moment. However, the idealized phrase is a helpful starting point. Future study will explore how classical composers employ harmonic progressions in larger musical works that combine multiple phrases (which are not self-sufficient) into larger themes and movements.

The idealized phrase

The idealized phrase (also called the phrase model) is a single musical phrase that progresses through an entire cycle of harmonic functions, beginning and ending on tonic. (Strict voice-leading exercises are such phrases.) These phrases begin with a point of stability (tonic), move away from that stable point, and then eventually lead to a point of high tension and resolution (an authentic cadence). This pattern of stability–instability–stability, or rest–motion–rest, with a single goal at the end, should be familiar both from species counterpoint and from strict keyboard-style voice-leading. (This pattern also governs large-scale formal structures in classical music.)

The simplest phrase that exhibits this complete harmonic cycle is a tonic-dominant-tonic progression: I–V–I. This phrase begins and ends with the most stable harmony (I), and includes an authentic cadence (V–I). The V is the high point of instability, containing the tendency tone (ti) that most strongly points to the final point of arrival (do, or tonic).

This harmonic cycle can be expanded by inserting a subdominant chord, a destabilized tonic chord, or both, as in the following examples:

I IV V I
I II V I
I VI V I
I VI II V I

In functional bass terms, any harmonic progression that follows the pattern

T1 → (Tx) → (S) → D5 → T1

can serve as the basis for a complete idealized phrase. (Harmonies in parentheses are optional.)

Phrases are seldom 3–5 chords long, however, and a harmonic function can be expressed by more than a single chord. Thus we can understand the harmonic functions not simply as chords, but as zones of varying length in a phrase, that can be created by a number of chords or short chord progressions. More generally, then, our idealized musical phrase contains a single progression of functional zones T → (Tx) → (S) → D → T, begins with T1, and ends with an authentic cadence (D5–T1).