B Diminished

So we talked about all the diatonic C Major chords except for B Diminished. Time to cover it now.

Diminished chords are comprised of a root, its minor third, and its diminished 5th. The distance between the minor third and the diminished fifth is another minor third, so the chord is composed of 2 minor thirds and 1 diminished fifth (and their inverses).

Open Position B Diminished

|-|i|-|-|-| F
|-|-|-|p|-| D
|-|-|-|-|-|
|-|-|-|r|-| F
|-|-|m|-|-| B
|-|-|-|-|-|
 0 1 2 3 4

You'll need to use your ring finger to mute the 3rd string when you strum this chord, so you don't hear the open G string. Diminished chords tend to have pretty nasty shapes, and even vanilla B dim is no different. If it's difficult, you can drop the high F with the index finger.

Harmonic Function: Leading Chord

B dim is diatonic to the key of C Major, using its 7th scale degree as the root note. Thus, it is called the vii or vii° chord. The ° indicates diminished, but you have to be careful - we are dealing with the diminished triad here, not the diminished seventh. The Diminished 7th chord has two variations - half and full. Half is indicated with a slash through the ° symbol.

The vii chord is called the Leading Chord. Harmonically, it has a dominant function.