A diminished chord is a triad built from a root, a minor 3rd and a diminished 5th: the 3rd sits three semitones above the root and the 5th sits six semitones above the root. C diminished is C, Eb and Gb (spelled C, D# and F#), and its narrow, unstable sound makes it want to resolve.
What is a diminished chord?
A diminished chord is the tensest triad in music. It is built from a root, a minor 3rd and a diminished 5th, which means both of its intervals are one semitone narrower than the corresponding intervals of a minor chord. C diminished is C, Eb and Gb, spelled C, D# and F# in ChordFlow's sharp-only system: the 3rd is three semitones above the root and the 5th is six semitones above the root. The diminished 5th, the tritone, is the most unstable interval in Western music, and it is what gives the chord its dark, uneasy, almost queasy character. Diminished chords are rare as standalone chords but essential as connectors. In any major key there is exactly one diminished triad, built on the 7th degree: in C major it is Bdim, B, D and F. Its job is to pull the music back to the tonic, and it does that job more urgently than any other chord, because the tritone inside it demands resolution. Diminished chords also appear as passing chords between two other chords, where their instability is the point: they create a moment of tension that makes the arrival chord sound more settled by contrast.
How diminished chords are built
A diminished triad stacks two minor 3rds. C to Eb is three semitones, Eb to Gb is three more, and the outer notes span six semitones, a diminished 5th. Compare that with a minor triad, which stacks a minor 3rd and then a major 3rd, giving an outer span of seven semitones, a perfect 5th. That single semitone difference in the 5th is what separates the two chords, and it is easy to get wrong: lowering only the 3rd of a major chord gives a minor chord, but lowering the 3rd and the 5th gives a diminished chord. The diminished 7th chord extends the pattern by stacking a third minor 3rd on top: C, Eb, Gb and Bbb, spelled C, D#, F# and A in the sharp-only system, with each note three semitones from the next. Because the notes are evenly spaced, a diminished 7th chord is symmetrical: any of its four notes can be the root, and the chord divides the octave into four equal parts. That symmetry is why diminished 7th chords are used to modulate, since the same chord can belong to several keys at once. The B diminished guitar chord and the Bdim7 guitar chord show the triad and its 7th extension, and the key of C major shows where Bdim lives.
Worked example — F# diminished. F# major is F#, A# and C#. Lower the 3rd, A#, to A, and lower the 5th, C#, to C, and you have F# diminished: F#, A and C. Check the gaps: F# to A is three semitones and F# to C is six semitones. F#dim is the vii chord of G major, so it resolves naturally to the G major guitar chord, where the F# rises to G and the C falls to B. The F# diminished guitar chord shows the shape, and the chord finder builds any diminished triad from any root.
How to practise diminished chords
Diminished chords are best practised in context, as the tense chord before a resolution, because on their own they sound strange and unfinished.
- Play Bdim to C, F#dim to G and C#dim to D, holding the diminished chord for one beat and resolving on the next.
- On piano, play a diminished triad in root position, then in first inversion, then in second inversion, noticing that all three sound equally unstable.
- Build the diminished triad in the key of G major (F#dim) and the key of D major (C#dim) and compare their spellings.
- On guitar, learn the B diminished guitar chord shape and move it up three frets: the chord is symmetrical, so the shape repeats every three frets with a new root.
- Use the interval calculator to check the diminished 5th above any root before you build the chord.
Common mistakes
- Lowering only the 3rd — Lowering only the 3rd of a major chord gives a minor chord, not a diminished chord. A diminished chord needs the 5th lowered as well, from seven semitones to six above the root.
- Using a diminished chord as a home chord — A diminished triad is unstable by design and cannot be the tonic of a song. If your progression keeps landing on a diminished chord, check that you have not mistaken it for a minor chord.
- Spelling the 5th as a sharp 4th — In a diminished triad the top note is a diminished 5th, not an augmented 4th, even though both span six semitones. C to Gb is a diminished 5th; C to F# is an augmented 4th. The letter name matters because it decides the chord's function.
Quick reference
For example, the diminished triad chord on C is C – D# – F#, and every other row follows the same pattern. The table below works out the diminished triad chord on six roots so you do not have to. Read down the notes column and notice how the root, minor 3rd and diminished 5th spacing never changes, even though the letter names do, and use the first row as the worked example you can check your own calculations against.
| Root | Diminished Triad notes |
|---|---|
| C | C – D# – F# |
| G | G – A# – C# |
| D | D – F – G# |
| A | A – C – D# |
| E | E – G – A# |
| F | F – G# – B |
Once the six rows feel familiar, transpose the table yourself: pick a root that is not listed, work out the notes of the diminished triad chord from the interval pattern, and play the result. That single exercise, repeated for a few roots, is what turns a table you have read into a skill you own.
Frequently asked questions
What notes are in a diminished chord?
The root, the minor 3rd and the diminished 5th. C diminished is C, Eb and Gb, spelled C, D# and F# in the sharp-only system. The 3rd sits three semitones above the root and the 5th sits six semitones above the root.
Why do diminished chords sound so tense?
Because they contain a tritone, the interval of six semitones between the root and the diminished 5th, which is the most unstable interval in Western music. The ear hears the tritone as a clash that needs to resolve, so the whole chord feels restless.
What is the difference between diminished and half-diminished?
A diminished triad has a diminished 5th and no 7th, while a half-diminished 7th chord adds a minor 7th to a diminished triad. Cm7b5 is C, Eb, Gb and Bb: the triad is fully diminished but the 7th is only minor, hence half-diminished.
Where do diminished chords appear in a key?
On the 7th degree of a major key and the 2nd degree of a natural minor key. In C major the diminished chord is Bdim; in A minor it is also Bdim. Its job in both cases is to pull toward the tonic.
Can I use a diminished chord as a passing chord?
Yes, and that is one of its best uses. A diminished chord placed between two chords a tone apart creates a smooth chromatic walk: for example, C, C#dim, Dm moves the bass up by semitones and adds tension that resolves into the Dm.