Definition
The half-diminished seventh chord is a diminished triad with a minor seventh: in C major, on the VII degree, B–D–F–A. In chord symbols it is written Bø7 or Bm7(b5). It has two main roles in tonal music:
- in major, on the VII degree (viiø7), it is a dominant chord without root (a G9 without the G);
- in minor, on the II degree (iiø7), it is a predominant chord (B–D–F–A in A minor).
The chord is called “half” diminished because only the triad is diminished, whereas the full diminished seventh chord would also have a diminished seventh (Ab).
viiø7 in major
- It resolves to the tonic: B → C, F → E, A → G, D → E (or C).
- Watch out for fifths: if the D is below the A (perfect fifth between the two voices), it must rise to E; otherwise D → C and A → G form parallel fifths. The tonic will then have the third doubled, acceptable in this context.
- More often it goes first to the V7: the seventh A falls to G as a passing note, the other notes stay (B–D–F–A → B–D–F–G), and then the V7 resolves to I.
- It is also used in inversions, especially viiø6/5 and viiø4/3.
iiø7 in minor
- In the minor key the ii is a diminished triad; with the seventh it becomes half-diminished: in A minor B–D–F–A.
- It is the most expressive predominant of the minor key: iiø6/5 – V – i (D in the bass, rising to E).
- The seventh (A) falls to the leading tone G#; the F falls to E; the B and D can stay as common tones of the V7.
The half-diminished as “minor sixth”
The first inversion of the half-diminished has the same notes as a minor chord with a sixth: B–D–F–A = Dm6 (D–F–A–B). This identity explains why the ivm6 of modal interchange and the half-diminished easily substitute for one another.
Practical rules
- In major use the viiø7 when you want a softer, “shadier” dominant than the vii°7.
- In minor, the iiø7 prepares the cadential six-four very well: iiø6/5 – i6/4 – V7 – i.
- In jazz the m7(b5) is the ii of the minor ii–V: Bm7(b5) – E7 – Am.
Common mistakes
- Confusing ø7 and °7: B–D–F–A and B–D–F–Ab have different functions and colors.
- Resolving the viiø7 with parallel fifths between D–A and C–G.
- Using the iiø7 in major without meaning to: in C major, D–F–Ab–C is a borrowing from the minor.
Examples
- Wagner, Tristan chord: F–B–D#–G# has the structure of a half-diminished chord, and has become the symbol of late Romantic chromaticism.
- Mendelssohn, Wedding March: after the opening trumpet fanfare a half-diminished chord appears, unexpected in such a festive piece.
- Chopin, Scherzo no. 1: opens with two violent chords, the first of which is a half-diminished.
- “Autumn Leaves”: F#m7(b5) – B7 – Em is the ii–V–i of the relative minor key.
- “Stella by Starlight”: the piece begins precisely with an m7(b5).
How to use it in composition
- The iiø7 gives the minor cadence a melancholy and intense color: ideal in ballads and film scores.
- In major, viiø7 – V7 – I creates a prolonged dominant with a descending motion of the top voice (A → G).
- As a piano voicing, the half-diminished in close position sounds soft; with the seventh on top it is tenser.
Exercise
- Write in D major: I – viiø6/5 – I and I – viiø7 – V7 – I, watching the fifths.
- Write in G minor: i – iiø6/5 – i6/4 – V7 – i.
- Play Dm6 and Bø7 at the piano and verify that they contain the same notes.
Connections
Seventh chords; The diminished seventh chord and its uses; Locrian #2 (half-diminished); The Tristan chord; Harmony in the minor key; The jazz ii–V–I.