A diminished triad is a three-note chord built from two stacked minor thirds, producing a minor third and a diminished fifth above the root. In AP Music Theory it is one of the four triad qualities (major, minor, diminished, augmented) you identify by ear and in notation under Topic 3.1.
A diminished triad is what you get when you stack two minor thirds on top of each other. Measure from the root and you find a minor third and a diminished fifth. That diminished fifth (also called a tritone) is the giveaway. It's the unstable, dissonant interval that makes the chord sound tense and like it needs to resolve somewhere.
In the CED's terms (PIT-1.O.1), a triad is a chord of three distinct pitches stacked in thirds, on adjacent lines or spaces. The diminished triad is one of the four possible qualities those stacked thirds can produce. Here's a quick mental shortcut. Start with a minor triad (minor third + perfect fifth) and shrink the fifth by a half step. That's a diminished triad. It shows up naturally on the leading tone in major keys (vii°) and on the second scale degree in minor keys (ii°), which is why it almost always feels like it's pulling toward another chord rather than sitting still.
This term lives in Unit 3: Music Fundamentals III - Triads and Seventh Chords, specifically Topic 3.1 Triad and Chord Qualities (M, m, d, A). It directly supports learning objective 3.1.A, which asks you to describe chord quality in both performed and notated music. That dual demand matters. You need to recognize a diminished triad when you see it on a staff (count the thirds, check the fifth) and when you hear it (that tight, unresolved, slightly anxious sound). The diminished triad is also the foundation for two seventh chords later in Unit 3, so if you can't build it quickly, the half-diminished and fully diminished seventh chords will feel like a wall.
Keep studying AP Music Theory Unit 3
Visual cheatsheet
view galleryChord quality (Unit 3)
Diminished is one of the four triad qualities (M, m, d, A) tested in Topic 3.1. Each quality is just a different recipe of stacked thirds, and diminished is the all-minor-thirds recipe. If you can name the two thirds in any triad, you can name its quality.
Diminished fifth (Unit 2)
The interval that defines this chord. A diminished triad's outer interval, root to fifth, is a diminished fifth (a tritone). Your Unit 2 interval skills are exactly what you use to verify a triad is diminished and not minor.
Diminished seventh chord and half-diminished seventh chord (Unit 3)
Both seventh chords start with a diminished triad on the bottom. Add a diminished seventh on top and you get a fully diminished seventh chord; add a minor seventh and you get a half-diminished seventh chord. The triad is the building block; the seventh on top decides which chord you have.
Harmonic function and the leading-tone chord (Unit 4)
In a major key, the triad built on scale degree 7 is diminished (vii°), and its instability is the point. The tritone inside it wants to resolve to the tonic, which is why diminished triads act like dominant-function tension chords in progressions.
Diminished triads show up in two main ways. First, identification questions, both visual and aural. You'll see or hear a triad and pick its quality, or get a stem like "Which triad type has a minor third and diminished fifth?" Second, comparison questions that test whether you actually know the interval recipes. Practice questions regularly ask how a diminished triad's structure differs from a minor triad's (same minor third on the bottom, but a diminished fifth instead of a perfect fifth) or from a major triad's (major third + perfect fifth versus minor third + diminished fifth). The skill you must perform is fast interval analysis. Spell the triad, measure root to third and root to fifth, and name the quality. On the aural side, train your ear to catch the tritone's tension; that's what separates diminished from minor in a listening question.
Both chords have a minor third above the root, so they're easy to mix up on paper. The difference is the fifth. A minor triad has a perfect fifth (its upper third is major), while a diminished triad has a diminished fifth (its upper third is also minor). One half step in the fifth completely changes the sound, from stable and dark (minor) to unstable and tense (diminished). Always check the outer interval, not just the bottom third.
A diminished triad is two minor thirds stacked together, giving a minor third and a diminished fifth above the root.
The diminished fifth (tritone) between the root and fifth is what makes the chord sound dissonant and unresolved.
It differs from a minor triad by only one note: the fifth is lowered a half step from perfect to diminished.
Diminished triads occur naturally as vii° in major keys and ii° in minor keys, which is why they usually pull toward resolution.
Topic 3.1 (LO 3.1.A) requires you to identify the diminished quality both in notation and by ear.
The diminished triad is the base of both the half-diminished and fully diminished seventh chords later in Unit 3.
It's a three-note chord built from two stacked minor thirds, so the root supports a minor third and a diminished fifth. It's one of the four triad qualities (major, minor, diminished, augmented) covered in Topic 3.1 of Unit 3.
No. The fifth is diminished, a half step smaller than perfect, and that interval is the whole reason the chord sounds tense. If the fifth is perfect and the third is minor, you're looking at a minor triad instead.
No. The triad has three notes; the diminished seventh chord adds a fourth note (a diminished seventh above the root) on top of the same triad. The triad is the foundation, and the seventh chord is the extended version you'll meet later in Unit 3.
Both have a minor third above the root, but a minor triad has a perfect fifth while a diminished triad has a diminished fifth. In thirds language, minor stacks minor + major, and diminished stacks minor + minor.
In a major key, the triad on scale degree 7 (the leading tone, vii°) is diminished. In a natural minor key, the triad on scale degree 2 (ii°) is diminished. Knowing these spots helps you predict chord quality without spelling every interval.
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