Diatonic Sequences

A diatonic sequence is a repeating harmonic or melodic pattern that transposes by a consistent interval within a single key, using only diatonic pitches. Because each repetition (called a "unit") moves the pattern to a new scale degree, the interval quality varies slightly from unit to unit (major thirds become minor thirds, perfect fifths become diminished fifths, etc.), but the overall contour and voice-leading logic remain the same. Sequences suspend normal functional harmony briefly, substituting momentum and pattern for goal-directed progression.

The most common diatonic sequences are: (1) the descending-fifth (or ascending-fourth) sequence, where roots move down by fifth — I–IV–viiº–iii–vi–ii–V–I — producing pairs of chords related by falling fifths; (2) the descending-third sequence, where each chord root falls a third, often combined with alternating 5–6 motion to avoid parallel fifths; and (3) the ascending-fifth (descending-fourth) sequence, less common but used for sequences of 5–6 or 7–6 suspensions. Each type generates its own characteristic bass line and suspensions.

Sequences are especially useful for traversing harmonic space quickly or for prolonging a single functional area across many measures. The descending-fifth sequence, for example, connects any starting chord back to the dominant through a chain of falling fifths, while the descending-third sequence can link I down to vi or IV in a graceful, stepwise-feeling descent. Analysts mark sequences with brackets and labels indicating the transposition interval (e.g., "Seq. ↓5th") to distinguish them from functional progressions. Understanding sequences is essential for Baroque and Classical counterpoint, where they underpin vast stretches of musical development.