A diatonic sequence is a harmonic pattern that repeats at a new pitch level while staying entirely in one key, like the descending-fifths progression I–IV–vii°–iii–vi–ii–V–I. In AP Music Theory, sequences are the accepted context for otherwise rare chords such as vi⁶ and iii⁶.
A diatonic sequence takes a short musical unit (a chord pair, a bass pattern, a voice-leading move) and repeats it at a different pitch level, over and over, without ever leaving the key. "Diatonic" is the key word. Every chord in the pattern is built from the notes of the home scale, so no accidentals are required and no modulation happens. The most famous example is the descending-fifths sequence, where each chord's root drops a fifth: I–IV–vii°–iii–vi–ii–V–I. That's every diatonic chord in the key, lined up like dominoes.
Here's why sequences get special treatment in AP Music Theory. Normal functional harmony has rules about where chords can go (predominants move to dominants, dominants resolve to tonic, and so on). Inside a sequence, the pattern takes over and those function rules relax. That's why chords you almost never write elsewhere, like vi⁶ and iii⁶, are acceptable in a sequence. The repeating pattern justifies them. Think of a sequence as harmony on autopilot. Once the pattern starts, the music rides it until it lands back on a functional chord.
Diatonic sequences live in the harmony and voice-leading units of the course (Units 4-5), where you learn standard chord progressions and then learn the sanctioned exceptions to them. The CED treats sequences as the legitimate home for mediant and submediant chords in inversion, which is exactly the kind of detail that separates a 4 from a 5 on part-writing tasks. Sequences also explain a huge amount of real repertoire. Baroque composers like Vivaldi and Bach built entire passages out of descending-fifths sequences, so recognizing one in a score excerpt instantly tells you what the next several chords will be. That predictive power is gold on harmonic analysis questions, because once you spot the pattern, you can label chords faster than you can hear them.
Keep studying AP® Music Theory Unit 6
Visual cheatsheet
view galleryCircle of fifths (Unit 1)
The descending-fifths sequence is the circle of fifths set in motion inside a single key. Instead of moving between keys on the diagram, the chord roots themselves travel by fifth: I down to IV, IV down to vii°, and so on. If you know the circle, you can predict every chord in the sequence.
Parallel fifths (Unit 4)
Sequences relax the rules about chord order, but they do not relax voice-leading rules. Root-position chords moving by fifth are a parallel-fifths trap, which is exactly why sequences often alternate inversions (like 6/3 chords) to keep the outer voices safe. Part-writing a sequence is a parallel-fifths obstacle course.
Functional harmonic progression (Units 4-5)
Normal progressions follow function: tonic, predominant, dominant, tonic. A sequence pauses that logic and substitutes pure pattern, then hands control back to function when it ends, usually landing on ii or V. Knowing where a sequence starts and stops tells you which chords need functional justification and which don't.
Sequences show up two main ways. In harmonic analysis (multiple choice with a score, or Roman numeral analysis on the FRQs), you might see a passage where the chords seem to break the normal progression rules. Spotting the repeating pattern is the answer; label the chords and trust the sequence. In part writing, a figured bass or Roman numeral FRQ can include a sequential passage, and there you have to keep the voice leading clean across every repetition of the pattern, watching especially for parallel fifths and octaves between repeating units. No released FRQ has hinged on the term "diatonic sequence" by name, but descending-fifths motion is a standard feature of the progressions you're asked to realize, and recognizing the pattern makes the whole problem faster and safer.
Both repeat a pattern at a new pitch level, but a melodic sequence is a single line restating a motive higher or lower (a Unit 6 melodic device), while a diatonic sequence is a harmonic pattern, a repeating unit of chords. They often happen together, since a chord sequence usually produces sequential melodies on top, but on the exam "sequence" in a melody question means the motive, and in a harmony question it means the chord pattern.
A diatonic sequence repeats a harmonic pattern at different pitch levels while staying entirely in one key, with no accidentals and no modulation.
The descending-fifths sequence (I–IV–vii°–iii–vi–ii–V–I) is the most common type, and it's just the circle of fifths running inside a single key.
Sequences are the accepted context for chords like vi⁶ and iii⁶ that you should avoid in normal functional progressions.
Inside a sequence, the repeating pattern overrides the usual chord-function rules, but voice-leading rules like avoiding parallel fifths still fully apply.
If you spot a sequence in a score excerpt, you can predict the next several chords, which speeds up Roman numeral analysis dramatically.
It's a harmonic pattern that repeats at a new pitch level while staying completely in one key. The classic example is the descending-fifths progression I–IV–vii°–iii–vi–ii–V–I, where every chord root drops a fifth.
Yes, but mainly inside a sequence. Chords like iii⁶ and vi⁶ are weak in normal functional progressions, and a diatonic sequence is the standard context where AP Music Theory accepts them, because the repeating pattern justifies them.
No. That's exactly what "diatonic" means here. Every chord uses only notes from the home scale, so there's no modulation and no accidentals. A sequence that adds chromatic notes or moves to a new key is a chromatic or modulating sequence, which goes beyond the diatonic version.
The circle of fifths is a map of all 12 keys and their key signatures from Unit 1. A descending-fifths sequence applies that same fifth-based motion to chord roots within one key, so the chords travel by fifth but the key never changes.
Absolutely. Sequences relax chord-order rules, not voice-leading rules. Root motion by fifth makes parallels especially likely, which is why sequences typically alternate root-position and first-inversion chords to keep the voices clean.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.