A diminished fifth is a dissonant interval spanning six half steps (e.g., B up to F). In AP Music Theory, it matters most in 18th-century voice leading, where a descending diminished fifth is an allowable bass-line leap only if it resolves properly, usually by step in the opposite direction.
A diminished fifth is an interval six half steps wide, like B up to F. Take a perfect fifth and shrink it by one half step, and you get this famously unstable sound. It is one spelling of the tritone, the interval that splits the octave exactly in half, and your ear hears it as tension that wants to resolve.
In the common practice style that AP Music Theory tests, the diminished fifth shows up in two big places. First, it sits between the leading tone and scale degree 4, which means it lives inside the V7 chord and the diminished triad (vii°). Second, it appears as a melodic leap in bass lines. The CED is specific here (PIT-3.D.2): allowable bass leaps include thirds, perfect fourths and fifths, sixths, and octaves, plus descending diminished fifths, but only if resolved properly. The resolution is the whole deal. A diminished fifth left hanging is a style error; a diminished fifth that resolves by step is good 18th-century writing.
This term lives in Unit 4: Harmony and Voice Leading I, specifically Topic 4.1. It supports learning objective 4.1.E (compose a bass line following normative 18th-century melodic procedures) because the descending diminished fifth is the one dissonant leap the style explicitly permits, with strings attached. It also feeds 4.1.A, since voice leading requires resolving tendency tones according to stylistic precedent, and the two notes of a diminished fifth (leading tone and scale degree 4) are exactly those tendency tones. When you write the bass line for the part-writing FRQs, knowing which leaps are legal and how to resolve them is the difference between earning points and losing them on style errors.
Keep studying AP Music Theory Unit 4
Visual cheatsheet
view galleryTritone (Unit 4)
The diminished fifth and the augmented fourth are the two spellings of the tritone. Same six half steps, same restless sound, but the spelling tells you which way the notes want to resolve.
Leading Tone (Unit 4)
The diminished fifth in a key is built between the leading tone and scale degree 4. That is why V7 sounds so pulled toward tonic. The leading tone rises to 1 while 4 falls to 3, and the dissonance collapses inward.
Diminished Chord (Unit 4)
Stack two minor thirds and the outer interval is a diminished fifth. That interval is what makes the vii° triad unstable and dominant-functioning, so it almost always resolves to tonic.
Melodic Interest (Unit 4)
Bass lines balance steps and leaps to stay interesting (PIT-3.D.1). The descending diminished fifth is the spiciest leap on the approved list, and using it correctly shows you can add drama without breaking style rules.
The diminished fifth shows up most directly in part writing. The 2025 SAQ Question 7 asked you to complete a bass line for a given melody following 18th-century voice-leading procedures, with Roman and Arabic numerals below. That is exactly where the leap rule applies. A descending diminished fifth in your bass is acceptable only if you resolve it properly, typically by stepping in the opposite direction afterward. Multiple-choice questions hit the same idea from the identification side. Expect stems like "which leaps are allowable in a bass line" or questions where a diminished fifth appears as an answer choice next to other awkward intervals, like the augmented second you get moving from raised-7 territory in harmonic minor. You also need it for error detection (4.1.A). If you see a bass voice leap a diminished fifth and then leap again in the same direction, flag it.
Both span six half steps and both are tritones, but the spelling changes everything in voice leading. A diminished fifth (like B-F) resolves inward to a third, while an augmented fourth (like F-B) resolves outward to a sixth. On the exam, count the letter names. B to F is some kind of fifth (B-C-D-E-F is five letters), so six half steps there must be a diminished fifth, not an augmented fourth. Spelling determines the interval name and the expected resolution.
A diminished fifth spans six half steps and is one spelling of the tritone, the most dissonant interval in common-practice harmony.
In 18th-century bass lines, a descending diminished fifth is an allowable leap, but only if it resolves properly, usually by step in the opposite direction.
The diminished fifth in any key sits between the leading tone and scale degree 4, which is why V7 and vii° both pull strongly toward tonic.
A diminished fifth and an augmented fourth sound identical but are spelled differently and resolve in opposite directions (d5 inward, A4 outward).
When the diminished fifth resolves, the leading tone rises to tonic and scale degree 4 falls to scale degree 3, following the tendency-tone rules in PIT-4.A.1.
A diminished fifth is an interval six half steps wide, like B up to F, made by shrinking a perfect fifth by one half step. It sounds tense and unstable, and in tonal music it almost always resolves inward by step.
Yes and no. Every diminished fifth is a tritone, but "tritone" also covers the augmented fourth, which has the same six half steps spelled differently. On the AP exam, the spelling matters because it changes the interval name and the expected resolution.
Yes, but only descending and only if it resolves properly. The CED lists descending diminished fifths among allowable bass leaps (alongside thirds, perfect fourths and fifths, sixths, and octaves), with the condition that the leap resolves correctly, typically by step in the opposite direction.
They are enharmonically equivalent (both six half steps) but spelled and resolved differently. B-F is a diminished fifth and resolves inward to a third; F-B is an augmented fourth and resolves outward to a sixth. Count the letter names to tell them apart.
The V7 chord contains a diminished fifth between the leading tone and scale degree 4, the two strongest tendency tones in the key. The leading tone pulls up to tonic and scale degree 4 pulls down to 3, so the dissonance resolves right into the tonic chord.
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.