A chord consisting of four notes, each a minor third apart from the next. Since it contains two tritones, it sounds horribly dissonant, but since lowering any of its tones a semitone yields a dominant seventh, it is quite easily resolved.

Since the chord is symmetric, there are only three distinct fully-diminished seventh chords (named arbitrarily):

• A^o7 contains the notes A C Eb Gb
• F^o7 contains the notes F Ab Cb Ebb
• C#^o7 contains the notes C# E G B

The most standard place to find a fully-diminished seventh chord is in a phrase or piece written in a harmonic minor scale. The seventh tone of harmonic minor is a minor third away from the second tone, which is in turn a minor third from the fourth tone, which is in turn a minor third from the sixth tone, which is again a minor third from the seventh. Again, since the chord shape is symmetric, you can build a fully-diminished seventh off the second, fourth or sixth tones as well.

You might be wondering right now, "Why is it called a fully-diminished seventh? Would I be correct in assuming that there's some kind of quasi-dimished seventh?" And indeed you would be, except that it's called a half-diminished seventh. I'll include a little blurb on chord naming conventions at some future time, but right now I'm too tired.