I thought I'd share what I find is a very interesting chord: the diminished 7th chord. If you really want to wrap your head around this chord and its usage, definitely go to YouTube (I found the Dutch guy with the white board pretty insightful). Basically, a B diminished 7th chord is: B, D, F and A flat. All notes are a step and a half apart. For me, the chord shape that is the clearest is D and B strings at the 3rd fret, G and E strings at the 4th fret. The G string, in this case is the root note. I don't play the low E or A. If you can free yourself of thinking of the lowest note being the root, then this B diminished chord turns into D, F and A flat diminished 7th as well. They are all these same notes! Slide this chord up a fret and you get C, D sharp, F sharp and A diminished 7th chords. Up another fret you get C sharp, E, G and A sharp diminished 7th chords. In just these three adjacent positions all twelve diminished 7th chords are represented. Move the chord up another fret and and the cycle starts over. So, the 12 note chromatic scale is simply divided into four notes, each a step and a half apart -- it's just so mathematical, it blew my mind! Now for when to use the diminished 7th -- always. I jest, it's the most unresolved chord there is, so it will always be followed by a major or minor chord. Just put the diminished 7th chord a half step down from the target (major or minor) chord. For example, the B diminished 7th chord will precede and introduce your C major or minor. Now, think of any of the four notes in the diminished 7th chord as the root. Then, the B diminished 7th chord can easily precede D sharp, F sharp and A major or minor chords, as well as C major or minor. I find it fascinating that one of the three adjacent diminished 7th chords can precede 24 chords! Or, you can move this chord a half step at a time up and down the fretboard for a damsel in distress/silent movie vibe. Or play it three frets apart up and down the fretboard for a dreamlike interlude vibe. I hope this makes some sense. Keep in mind that I'm not talking about the half diminished, nor the minor 7th flatted fifth. In my mind, those are different. Enjoy, and practice, practice, practice.