Unequal fifths occur when the same two voices move from a diminished fifth to a perfect fifth, a motion avoided in 18th-century voice leading. On the AP Music Theory exam, they count as a voice-leading error in part writing and error-detection questions (Topic 4.1).
Unequal fifths happen when the same pair of voices forms a diminished fifth on one chord and then a perfect fifth on the next. The two voices move in similar motion, but the fifth changes quality, which is why it's "unequal" rather than "parallel." In 18th-century style, the move from d5 to P5 is the problem child. Going the other direction (P5 to d5) is much more forgivable, because the diminished fifth that results is a dissonance that wants to resolve, and it usually does.
Why is d5 to P5 a problem? A diminished fifth almost always involves tendency tones, usually the leading tone (ti) against fa. Those notes want to collapse inward to a third (ti up to do, fa down to mi). When they expand out to a perfect fifth instead, the voices ignore their tendencies and end up sounding like sloppy parallels. Per the CED (PIT-4.A.1), voice leading should resolve tendency tones according to stylistic precedent and keep voices independent. Unequal fifths break both expectations at once.
Unequal fifths live in Topic 4.1, Harmony and Voice Leading I (Unit 4), under learning objective 4.1.A, which asks you to identify and apply 18th-century voice-leading procedures through score analysis, error detection, writing exercises, and contextual listening. This is one of the specific errors you're expected to catch and avoid. It also matters for 4.1.B and 4.1.E (composing a bass line under a given soprano), since a poorly chosen bass note can create unequal fifths against the soprano even when the harmony itself is fine. Knowing this rule is part of what the CED means by emulating common-practice voice leading, alongside avoiding parallel fifths and resolving tendency tones correctly.
Keep studying AP® Music Theory Unit 4
Visual cheatsheet
view galleryParallel fifths (Unit 4)
Parallel fifths are P5 to P5; unequal fifths are d5 to P5. They're cousins. Both involve two voices moving in similar motion between fifths, and both get flagged as errors, but the interval quality is what separates them. Exam questions love making you tell them apart.
Leading Tone (Unit 4)
The diminished fifth in an unequal-fifths error almost always contains the leading tone against scale degree 4. If you resolve those tendency tones properly (ti up, fa down), the voices contract to a third and the error literally can't happen. Fixing tendency-tone resolution fixes unequal fifths for free.
Contrary Motion (Unit 4)
Unequal fifths require similar motion between the two voices. Moving the voices in contrary or oblique motion instead is the standard repair, which is the same logic you use to avoid parallel fifths and octaves.
Melodic Interest (Unit 4)
Don't confuse the harmonic d5 with the melodic one. Per PIT-3.D.2, a bass line may leap down a diminished fifth as a melodic interval if it resolves properly. Unequal fifths are a different thing entirely, a vertical interval between two voices, not a leap within one voice.
Unequal fifths show up in error-detection MCQs and in the scoring of the part-writing free-response questions. The 2025 exam used the concept in free-response Questions 5 and 6, where voice-leading errors like this cost points when you realize a figured bass or write from Roman numerals. In multiple choice, expect a stem that describes or notates two voices moving between fifths and asks you to name the error. Here's the trap to watch for. In a question like "the soprano moves D5 to E5 while the bass moves G3 to A3 in G major," both intervals are perfect fifths, so the answer is parallel fifths, with unequal fifths sitting there as a tempting distractor. You have to actually check the quality of each fifth, not just notice that two fifths happened in a row.
Both errors involve the same two voices forming fifths on consecutive chords in similar motion. The difference is quality. Parallel fifths are perfect fifth to perfect fifth. Unequal fifths are diminished fifth to perfect fifth (the qualities are unequal, hence the name). Quick test: measure both intervals. If both are perfect, it's parallel; if the first is diminished and the second is perfect, it's unequal. Note that P5 moving to d5 is generally acceptable in this style, so direction matters too.
Unequal fifths occur when the same two voices move from a diminished fifth to a perfect fifth, and this motion is avoided in 18th-century voice leading.
Moving from a perfect fifth to a diminished fifth is generally acceptable; it's the d5-to-P5 direction that counts as an error.
The diminished fifth usually involves the leading tone and scale degree 4, so resolving those tendency tones correctly (ti up, fa down) prevents unequal fifths automatically.
Don't confuse unequal fifths with parallel fifths: parallel fifths are P5 to P5, while unequal fifths change quality from diminished to perfect.
A descending diminished-fifth leap within a single bass line is a legal melodic move (if resolved properly) and has nothing to do with the harmonic error of unequal fifths.
On the part-writing FRQs, unequal fifths are scored as voice-leading errors, so check interval quality between voice pairs whenever fifths appear on consecutive chords.
Unequal fifths happen when the same two voices form a diminished fifth on one chord and a perfect fifth on the next, moving in similar motion. It's a voice-leading error in 18th-century style, tested under Topic 4.1 (learning objective 4.1.A).
No. Only the move from a diminished fifth to a perfect fifth is treated as an error. Going the other way, P5 to d5, is generally acceptable because the resulting diminished fifth is a dissonance that resolves normally.
Parallel fifths are perfect fifth to perfect fifth between the same two voices. Unequal fifths are diminished fifth to perfect fifth, so the qualities don't match. Both are errors, but exam questions often use one as a distractor for the other, so always check interval quality.
Resolve your tendency tones. The diminished fifth almost always contains the leading tone and scale degree 4, and if ti goes up to do while fa goes down to mi, the voices contract to a third instead of expanding to a perfect fifth. Contrary or oblique motion between the voices also eliminates the problem.
Yes, descending, and only if it resolves properly. Per the CED (PIT-3.D.2), a descending diminished fifth is an allowable bass leap. That's a melodic interval within one voice, which is completely separate from the harmonic error of unequal fifths between two voices.
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