What is a diminished triad?

A diminished triad is a three-note chord built from a root, a minor third (b3), and a diminished fifth (b5). It is constructed by stacking two minor thirds (3 + 3 semitones). It sounds tense and unstable, naturally wanting to resolve to another chord.

Where do diminished triads appear?

They appear naturally as the vii degree chord in major keys (e.g., Bdim in C major) and as the ii degree in minor keys. They are also widely used as chromatic passing chords in jazz, classical, and film scoring.

Diminished Triads on Guitar

In previous articles we looked at major triads and how they help you find your harmonic space on guitar. Diminished triads are the third type of triad, and they have a very distinctive sound: tense, unstable, and wanting to resolve.

A diminished triad is built from a root (R), a minor third (b3), and a diminished fifth (b5). Its interval formula is R - b3 - b5. They appear naturally as the vii degree chord in any major key. For example, in C major the seventh degree is B diminished (Bdim), made up of the notes B - D - F.

The Interval Formula

A diminished triad is built by stacking two minor thirds: 3 semitones + 3 semitones = 6 semitones between the root and the fifth. Those 6 semitones form a tritone, the most unstable interval in tonal harmony, and that is what gives the diminished triad its characteristic tension.

Let's compare the three basic triads:

Major: R - 3 - 5 (4 + 3 semitones)
Minor: R - b3 - 5 (3 + 4 semitones)
Diminished: R - b3 - b5 (3 + 3 semitones)

Notice that both major and minor triads span 7 semitones between root and fifth (a perfect fifth), while the diminished triad only spans 6 (a diminished fifth, or tritone). That one-semitone difference changes everything.

Where Diminished Triads Appear

Diminished triads are not exotic — they occur naturally in the harmony of every key:

vii degree in major keys: In C major, the diatonic chords are C - Dm - Em - F - G - Am - Bdim. Bdim is the natural seventh degree. You can see every degree in any key with the scale harmonizer.
ii degree in minor keys: In A minor, the diatonic chords are Am - Bdim - C - Dm - Em - F - G.
Chromatic passing chords: Very common in jazz and classical music. For example, the progression C - C#dim - Dm uses C#dim as a chromatic passing chord between C and Dm.

You'll hear diminished triads frequently in jazz, classical music, and film scores, where their tension is used to create dramatic moments or smooth transitions between chords.

Example Progression with Bdim

Let's use a progression that features the diminished chord naturally: C - Am - Bdim - C (I - vi - vii° - I). The Bdim acts as a leading-tone chord that resolves back to the tonic. Here are the full chord shapes:

Play this progression and listen to how the Bdim creates tension that wants to resolve to C major. That feeling of "I need to go somewhere" is the essence of the diminished triad.

CAGED-Derived Shapes for Bdim

Just as with major triads, we can find diminished triads in different areas of the neck. The notes of Bdim are B (R), D (b3), and F (b5). Here are several useful positions:

Extracting Diminished Triads

Starting from the full Bdim shape, we can extract compact triads on groups of three adjacent strings, just as we did with major triads.

The first row shows the full Bdim shape at position 2 and the two triads we can extract: one on strings 4-3-2 (F - B - D = b5 - R - b3) and one on strings 3-2-1 (B - D - F = R - b3 - b5). The second row shows another inversion at position 7 on strings 3-2-1 (D - F - B = b3 - b5 - R).

Practical Tips

Chromatic passing chord: Use a diminished triad to connect two chords a whole step apart. For example, C - C#dim - Dm sounds smooth and elegant.
Leading-tone chord: The vii° resolves naturally to I. Try playing Bdim before returning to C major in any progression.
Symmetry every 3 frets: Because the diminished triad is built by stacking minor thirds (3 semitones), the same triad shape repeats every 3 frets. If you know one Bdim position, move it up 3 frets and you'll have another inversion of the same chord (Bdim = Ddim = Fdim in terms of notes with enharmonic respelling).
Chromatic approach: Play a diminished triad one semitone below your target chord for a very effective resolution.

Next Steps

With diminished triads you now have a powerful harmonic tool for creating tension and chromatic movement. Review major triads or minor triads if you need to refresh the fundamentals. In upcoming articles we'll explore dominant seventh chords and how triads combine with them to create even richer progressions.