Harmonic Sequences

A harmonic sequence is a short pattern of chords that gets repeated, each time starting a little higher or a little lower. You play a small group of chords, then play the same shape again shifted up or down, then maybe once more. The pattern stays the same; only its pitch level moves.

It is like saying the same short phrase again on a higher step, and then a higher step again, climbing or descending. Because the ear recognizes the repeating shape, a sequence feels both familiar and moving, like walking up or down a staircase of chords.

Play any two chords, then play the same two chords starting one note higher, then one note higher again. That climbing repetition is a sequence.

A harmonic sequence applies to chords a principle at work everywhere in music: repetition at a new pitch level. Restating a pattern a step or a fifth away lets a passage move forward while staying unified, since the ear registers the same model at each new pitch, so the music grows longer while staying built from one recognizable unit. It extends a phrase with very little new material.

The model-and-copies structure is what gives a sequence its particular logic. A composer states a unit, transposes it by a fixed interval, and repeats, so the listener locks onto the pattern and follows it through pitches that on their own might sound arbitrary. That predictability is the source of a sequence's drive, and the parts ahead sort sequences by how their roots or bass move.

The most common harmonic sequence moves by falling fifths. Each chord's root is a fifth below the one before, chord after chord. Because a root falling a fifth is the strongest chord change there is, chaining several of them makes a strong, rolling motion.

You can hear it as one resolution after another, each chord seeming to lead firmly into the next. This falling-fifths pattern drives long stretches of baroque music especially, spinning out a passage through a whole chain of chords that all pull downward toward home.

Play a chord, then the chord a fifth below it, then a fifth below that, and keep going. That tumbling chain is a descending-fifths sequence.

Descending fifths is the most common sequence, and it can run a long way. Let each root fall a fifth, then again, and the roots move through the diatonic collection in a fixed order, so the ear locks onto the model after a statement or two and follows it through the whole chain. The interval stays a fifth every time (in the diatonic chain one of them is a diminished fifth), which is what lets the pattern run so long without wandering.

Laying the circle of fifths out in a line is one way to see what the sequence does. The same fifth relationship that organizes the keys also organizes chords within a key, so a diatonic descending-fifths sequence walks through the key's chords in that fixed order. A single such sequence can run the full chain I, IV, vii diminished, iii, vi, ii, V, I.

Not every sequence moves by fifths; many move by step, with the pattern climbing or descending one scale note at a time. An ascending stepwise sequence walks the pattern up the scale; a descending one walks it down. These give a smoother, more gradual rise or fall than the bigger leaps of a fifths sequence.

A favorite trick makes these sound especially smooth: the chords take turns, one in its basic position and the next one inverted, as they step along. That alternation keeps the voices moving neatly and avoids clumsy parallel motion, so the sequence glides up or down the scale.

Play two chords, then move that pair up one scale step, then up another step. That stepwise climb is an ascending sequence.

Stepwise sequences solve a specific voice-leading problem that fifth sequences avoid by their nature. Moving a whole chord up or down by step tends to drag all the voices in parallel, producing the parallel fifths and octaves that tonal counterpoint forbids, so a plain stepwise chain of root-position chords quickly sounds crude. The 5-6 alternation exists to break up those parallels.

By alternating root-position chords with first-inversion chords, the 5-6 pattern shifts one voice off the parallel track at every step, so the bass can walk smoothly through the scale while the upper voices stay independent. The device is old and widespread, a standard piece of contrapuntal craft. When root position trades with first inversion over a stepwise bass, a 5-6 sequence is at work, and that alternation is its signature.

Sequences come in two flavors. A diatonic sequence uses the notes of the key, so as the pattern repeats a step lower, each copy bends a little to fit the scale. It stays settled in one key. Sometimes composers raise a note or two so that each copy leans into the next chord, but the pattern still walks down through the same key.

A chromatic sequence copies the pattern exactly, every step the same size, so it needs sharps or flats from outside the key, and it can wander away from home as it goes. It sounds more restless and colorful.

Play a two-chord pattern using only white keys, moving it down the scale. That all-white-key version is a diatonic sequence.

The choice between diatonic and chromatic sequence is a choice about how strictly the pattern is copied. A diatonic sequence measures its transposition in scale steps, so the literal size shifts from copy to copy: a perfect fifth in one statement lands as a diminished fifth in the next. Secondary dominants can sharpen each unit's pull toward the next chord without changing that plan.

A chromatic sequence transposes the model by a literal interval and copies it exactly, so the chord types stay fixed and the copies step outside the key. Because each exact copy implies its own key, a chromatic sequence can carry a passage away from home one unit at a time.

Sequences are one of a composer's favorite tools because they do so much at once. They build momentum, since the repeating pattern pushes the ear forward. They fill space, extending a short idea into a long passage. And they hold a passage together, because the same shape keeps returning even as the pitch climbs or falls.

A sequence usually plays its pattern a few times and then moves on. It does not sound boring, despite the repeating, because the pattern keeps moving to new notes. Your ear enjoys recognizing the shape while still being carried somewhere new, which is a big part of why sequences turn up in so much music, from Bach to film scores.

Listen for a spot where a bit of music seems to repeat while climbing or descending. You have found a sequence doing its work.

Sequences are among the strongest tools a composer has for spinning a small idea into a large structure. A single short model, transposed a few times, can generate an entire passage that stays unified because one shape underlies it, which is why sequential technique underlies so much development and extension in tonal music, from a two-bar continuation to a long drive toward a distant key.

Their effectiveness rests on how the ear handles patterned repetition. Once a listener grasps the model and its interval of transposition, they predict the pattern and follow it through harmony that would otherwise seem to wander, so the sequence licenses motion the ear might not accept as free progression. Two things set its limit: the interval of transposition and how many times the model repeats. Most sequences state the model a few times, then break off.