The terms “fully-diminished” and “diminished 7th” are used interchangeably. For the purpose of this explanation, I will be using only the term “diminished 7th”, because he abbreviation for this chord is “dim7″.
- A diminished chord is always a four note chord; when we see only three notes, a triad for example, it is incomplete. Following this reasoning, C dim (C Eb Gb) is a triad omitting the top note, while C dim7 (C Eb Gb Bbb) is the full chord or “fully” diminished chord. I have followed this reasoning, since it is more exact.
- A diminished chord is either a three or four note chord. Following the latter reasoning, C dim means either C Eb Gb or C Eb Gb Bbb. The advantage to this way of thinking are that the label is simpler, also allowing greater flexibility.