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The problem it solves
The diminished triad brings the tension tonal music needs to create direction and resolution. It appears naturally on the leading tone (vii°) and pushes toward the tonic.
Detailed theory
Key idea
It stacks two minor thirds in a row (3 + 3 semitones).
Between the root and the fifth there is a diminished fifth (six semitones, a tritone): that is what makes it unstable and tense.
Understand it
Start from a root and stack a minor third (three semitones): that is the third. On top put another minor third (three more semitones): that is the fifth. Two equal thirds, both minor.
In C: C (root), Eb (minor third over C) and Gb (minor third over Eb). From C to Gb there are six semitones: a diminished fifth, also known as the tritone.
That lowered fifth is the key to its character. The perfect fifth of the major and minor triad gave stability; the tritone, instead, sounds unstable and tense, and makes the chord "ask" to resolve onto another.
The diminished triad is the chord that forms on the seventh degree (the leading tone) of a major scale: in C major it is B-D-F, the vii° chord. That is why it often works as a bridge that pushes toward the tonic.
An analogy: if the major and minor triads were well-plumbed towers, the diminished one is a slightly leaning tower. It stands for a moment, but the whole thing points toward where it wants to fall: toward resolution.
Stacked triad
B diminished (B-D-F): the diminished triad on the leading tone, the vii° chord of C major that pushes toward the tonic.
How to recognise it
How it's written
It is written by stacking 1-b3-b5 over the root, for example C-Eb-Gb. In chord symbols it is shown with the ° sign or with dim: C° and Cdim mean C diminished.
How it feels
It sounds unstable and tense, as if left hanging. The diminished fifth (the tritone) gives it a push that demands resolving; it does not rest like the major or the minor.
Common mistake
Confusing the diminished fifth with a perfect fifth: in the diminished triad the fifth is lowered a semitone (six semitones instead of seven).
Expecting the diminished triad to sound stable: its function is precisely to create tension and push toward a resolution.
Try it
Build C diminished by stacking two minor thirds: C, Eb, Gb. Count the semitones (3 + 3) and check that from C to Gb there are six.
Play B-D-F (the vii° of C major) and then resolve it to C major: you will feel the diminished tension relax as it reaches the tonic.
On the instrument
Stacked triad
C diminished: two minor thirds stacked (C-Eb, 3 semitones, and Eb-Gb, 3 semitones), with a diminished fifth between C and Gb.
Interval distance
C-Gb: the diminished fifth (6 semitones, a tritone). It is the interval that gives the chord its characteristic tension.
Where it's used
- Creating tension
- Introducing an unstable chord (the tritone) that asks to resolve onto another.
- Harmonising the leading tone
- Playing the seventh-degree chord (vii°) that pushes toward the tonic in a major key.
- Recognising the tritone
- Identifying by ear the diminished fifth that sets the diminished triad apart from the minor.
Examples
Stacked triad
G diminished (G-Bb-Db): the same structure of two minor thirds from another root. Play it as an arpeggio and as a chord.
Exercises
Play diminished triads — basic
Play the diminished triad shown, with simple roots (C, G, F).
Complete 5 attempts · 70% accuracy to pass
Play diminished triads — intermediate
Play the diminished triad shown, now with more roots (includes Bb).
Complete 8 attempts · 70% accuracy to pass
Play diminished triads — advanced
Play the diminished triad shown from any of the 12 roots, including notes with accidentals.
Complete 10 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/8 answeredQuestion 1/8
Which three notes make up a diminished triad?