What Is a Half-Diminished 7th Chord?

Half diminished vs fully diminished: three chords compared

The word diminished appears in three chord names. All three start from the same diminished triad, root, minor 3rd and diminished 5th.

The only difference between the half-diminished seventh chord and the fully diminished seventh chord is the top note: A in the half-diminished, A♭ in the fully diminished. One fret, very different sound. The half-diminished is darker than a minor 7th but still smooth; the diminished 7th is tense and symmetrical.

Another way to see the half-diminished chord: it is a minor 7th chord with the 5th lowered a half step. Bm7 is B D F♯ A; flatten the F♯ and you have Bm7♭5. That is where the name m7♭5 comes from.

Half-diminished shapes on guitar

• Bm7♭5 (x 2 3 2 3 x): root on the A string. The most used shape; move it up the neck for any m7♭5 with the root on the A string.
• Am7♭5 (x 0 1 0 1 x): the same shape at the open position, with the open A as the root.
• Em7♭5 (x x 2 3 3 3): root on the D string, top four strings only.
• F♯m7♭5 (2 x 2 2 1 x): root on the low E string. Mute the A string with the side of the finger on the low E.

The F♯m7♭5 shape is movable too: at fret 7 it gives Bm7♭5, and at fret 3 Gm7♭5. Neither shape uses the high e string: let the underside of a fretting finger rest on it so it stays quiet.

Where the half-diminished chord appears

Build 7th chords on every degree of a major scale and the 7th degree comes out half-diminished:

In a minor key the same chord is the ii chord. A minor uses the same notes as C major, so its ii chord is Bm7♭5. That gives the minor-key version of the ii–V–i cadence, where the V is usually made a dominant 7th:

Bm7♭5, E7, Am is the sound of minor-key jazz standards and of many minor ballads. Each chord moves to the next by small steps, and E7 brings the G♯ that leads up to A. The key of A minor shows its other chords.

Diminished triads and diminished 7th chords

The diminished triad is the chord on the 7th degree of a major key, such as Bdim in C major. On guitar it is usually played as a small three- or four-string shape:

The diminished 7th is symmetrical. Its four notes are all three frets apart (B, D, F, A♭), so moving the shape up three frets gives the same four notes again, just in a different order. In practice that means one shape and its copies three frets up cover every diminished 7th chord: there are only three different sets of notes.

How diminished 7ths are used

The diminished 7th is a passing chord. It links two chords whose roots are a whole step apart by filling the half step between them:

C♯dim7 is C♯ E G B♭. Three of its notes sit a half step from a note of Dm (C♯ to D, E to F, B♭ to A), which is why the bass line C, C♯, D sounds so smooth. The same trick works between G and Am (G♯dim7) or F and G (F♯dim7).

Augmented chords, the other altered triad

Old chord guides often cover diminished and augmented chords together. An augmented chord raises the 5th instead of lowering it: 1 3 ♯5. C augmented (C+ or Caug) is C E G♯. Like the diminished 7th it is symmetrical: its notes are four frets apart, so the shape repeats every four frets. It is used mainly as a passing chord between a major chord and its IV or vi, such as C, C+, F.

How to start using them

• Swap the vii chord of a major key for its m7♭5. In C, play Bm7♭5 instead of Bdim; it is easier to finger and fuller.
• In a minor song, try ii–V–i: Bm7♭5, E7, Am in A minor; F♯m7♭5, B7, Em in E minor.
• Between two chords a whole step apart, slip in the diminished 7th on the note in between.

Compare the whole 7th-chord family, including how these relate to dominant, major and minor 7ths, in what are 7th chords.