In AP Music Theory, a diminished interval is an interval one half step smaller than a perfect or minor interval of the same size, such as a diminished fifth (C to Gb) or diminished seventh; it sounds dissonant and is enharmonically related to other intervals like the augmented fourth (PIT-1.L.1).
A diminished interval is what you get when you shrink a perfect or minor interval by one half step without changing the letter names. Take a perfect fifth, C up to G. Lower the G to Gb (or raise the C to C#) and you now have a diminished fifth. Same idea with minor intervals. A minor seventh, C up to Bb, becomes a diminished seventh when you shrink it to C up to Bbb. The letter names matter because interval size is counted by letter, so C to Gb is a fifth even though it spans the same six half steps as C to F#.
The CED (PIT-1.L.1) lists diminished as one of the five interval qualities you describe alongside major, minor, perfect, and augmented. It also flags the classic trap. Intervals that sound identical but are spelled differently are enharmonic equivalents, and the diminished fifth (D up to Ab) versus the augmented fourth (D up to G#) is the CED's own example. Both are the tritone, the famously unstable six-half-step interval. Your ear hears one sound, but the spelling on the page decides which name is correct.
Diminished intervals live in Topic 2.5 (Interval Size and Quality) and support learning objective AP Music Theory 2.5.A, which asks you to describe the size and quality of intervals in both performed and notated music. That dual demand is the whole game. You need to spell a diminished fifth on staff paper and recognize its tense, unstable sound by ear. The foundation comes from Topic 1.3 (AP Music Theory 1.3.A), because counting half steps is how you verify quality. A diminished fifth is six half steps; if you can't count semitones reliably, you can't tell d5 from P5. Diminished intervals also set up later harmony, since the leading tone's pull toward tonic is built on the diminished fifth inside dominant-function chords.
Keep studying AP Music Theory Unit 2
Visual cheatsheet
view galleryAugmented Interval (Unit 2)
Augmented and diminished are mirror operations. Augmented stretches a perfect or major interval by a half step; diminished shrinks a perfect or minor one. If you can do one, you can do the other by reversing the move.
Enharmonic Equivalent (Unit 1)
The diminished fifth and augmented fourth are the CED's go-to example of enharmonic equivalents. They sound identical (both are the tritone) but the written letter names decide which label is correct, so always count letters before you count half steps.
Half Step (Unit 1)
Every quality judgment comes down to semitone counting from Topic 1.3. A perfect fifth is seven half steps, so a diminished fifth is six. Quality is just arithmetic once you know the perfect or minor benchmark.
Minor Interval (Unit 2)
Minor intervals are the doorway to diminished ones for 2nds, 3rds, 6ths, and 7ths. The chain goes major, then minor, then diminished, each step one half step smaller. Perfect intervals skip straight to diminished with no minor stop in between.
Multiple-choice questions hit diminished intervals two ways. Notation-based stems give you two written pitches and ask for size and quality, where spellings like C to Gb versus C to F# test whether you count letter names first. Aural stems play an interval and ask you to identify it, and the tritone's grinding dissonance is your tell. Practice questions in this area frequently quiz the basic rule itself, like how a diminished interval differs from an augmented one. Interval quality also feeds the written FRQs. The 2025 SAQ 7 bass-line completion required eighteenth-century voice-leading procedures and Roman numerals, and that kind of part-writing depends on handling diminished intervals correctly, like resolving the diminished fifth in dominant-function harmony and avoiding awkward melodic leaps.
Both alter a base interval by one half step, but in opposite directions. Diminished means one half step smaller than perfect or minor; augmented means one half step larger than perfect or major. The trap is enharmonics. D up to Ab (diminished fifth) and D up to G# (augmented fourth) sound exactly the same, so on a listening question you can't tell them apart by ear. On a notated question, the spelling alone determines the answer.
A diminished interval is exactly one half step smaller than a perfect or minor interval with the same letter-name size.
Count letter names first to get the size, then count half steps to get the quality; C to Gb is a fifth, not a fourth, even though it sounds like C to F#.
The diminished fifth and the augmented fourth are enharmonic equivalents, and both are the tritone, the CED's signature example of identical sound with different spelling.
Perfect intervals (unison, fourth, fifth, octave) go straight from perfect to diminished, while seconds, thirds, sixths, and sevenths pass through minor first.
Diminished intervals sound dissonant and unstable, which is why the diminished fifth inside dominant harmony pulls so strongly toward resolution in voice leading.
A diminished interval is an interval one half step smaller than a perfect or minor interval of the same size. For example, shrinking the perfect fifth C to G down to C to Gb makes a diminished fifth, which spans six half steps.
They sound identical (both are the six-half-step tritone) but they are not the same interval on paper. D up to Ab is a diminished fifth and D up to G# is an augmented fourth, and the AP exam grades you on the written spelling.
A minor interval is one half step smaller than a major interval, while a diminished interval is one half step smaller than a minor or perfect interval. So a diminished seventh (like C to Bbb) is a half step smaller than a minor seventh (C to Bb).
No. Unisons, fourths, fifths, and octaves are never major or minor. Shrink a perfect interval by one half step and it goes directly to diminished, which is a common multiple-choice trap.
Count the letter names to get the size, then count half steps to check quality against the perfect or minor benchmark. A perfect fifth is seven half steps, so any fifth spanning six half steps, like B up to F, is diminished.
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