Diminished (dim, °, dim7, °7) and Half‑Diminished (m7b5, -7b5, ∅)

Difference

The difference between a diminished chord and a half‑diminished chord lies in the seventh.

  • The half‑diminished chord (m7b5) has a minor 7th (7) — in other words, it’s an m7 chord with a diminished fifth (♭5).
  • The diminished chord has a diminished 7th (dim7, °7). Lowering the minor 7th by another semitone yields the diminished 7th, which is enharmonically equivalent to a major 6th (6) — only the theoretical naming differs.

Common Ground

Both are built on a diminished triad. This triad stacks two minor thirds and thus corresponds to a minor triad with a diminished fifth (♭5)* (recap: a minor triad stacks a minor third + major third, which together form a perfect fifth).

*) You could, in theory, write mb5 instead of dim or ° as a chord symbol — however this is uncommon.

In practice, a diminished chord almost never appears as a plain triad.
dim (°) usually implies dim7 (°7).

Chord Shapes

Drag the slider under the chord diagram(s) to the right to change each half‑diminished chord into a diminished chord.

The minor 7th of C is Bb. Strictly speaking, the diminished 7th is Bbb. Whether you prefer to think of (or write) it as A is up to you.

It’s similar with Cb or Fb. In everyday usage you might simply say “B” or “E”.

A diminished 7th chord is symmetric (stacked minor thirds), so C°7, Eb°7, Gb°7, and A°7 share the same notes.

Big difference in usage

The half‑diminished chord (m7b5) is a diatonic chord of the major scale. It sits on the 7th scale degree*. That means a Bm7b5 e.g. can be used in a song in C major (respectively A minor).

*) which corresponds to the 2nd degree in the parallel minor key.

The diminished chord (dim7, °7) is not part of the major or minor scale.

Examples

Half diminished (m7b5)

One of the most common uses of m7♭5 is the 2–5–1 progression in minor. Here’s an example in A minor (parallel minor of C major). Bm7♭5 is the diatonic chord on the 7th scale degree of C major — hence it’s the 2nd degree in the parallel minor key, A minor.

E7 is the 5th degree in A minor and resolves to the 1st degree, Am (that’s why we can change Em7 to E7 here).

Diminished (°7)

A diminished (°7) chord often functions as a passing/approach chord between degrees of the major/minor scale.