Diminished (dim, °, dim7, °7) and Half Diminished (m7b5, -7b5, ∅)
The difference between a diminished chord and a half diminished chord lies in the seventh.
- The half diminished chord (m7b5) has a minor seventh (7) - it's a m7 chord wth a flat five (b5).
- The diminished chord has a diminished seventh (dim7, °7). The minor 7th is again decreased by a semitone and hence is equivalent to a sixth (6) - only the theoretical approach is different.
Both are based on a diminished triad. This triad consists of two stacked minor thirds and thus represents a minor triad with a diminished fifth b5* (remember: the minor triad consists of a stack of minor third + major third, which together gives a perfect fifth).
*) You might consider writing mb5 as chord symbol instead of dim or °. However, this is uncommon.
Practically the diminished chord comes hardly ever as a pure triad.
dim (°) usually means dim7 (°7).
Drag the slider under the chord shape to the right to change each half diminished chord to a diminished chord.
The minor 7th of C is a Bb. It's up to you if you prefer thinking or writin an A instead.
It's similar with Cb or Fb. In daily life you may sometimes just say "B" or "E".
The diminished 7th chord is completely symetric. It can be seen as a stack of minor thirds. That means that C°7, Eb°7, Gb°7 and A°7 are made of the same tones!