B Diminished
dim
0 - 3 - 6Root + minor 3rd + diminished 5th
About B Diminished
B diminished is the chord the key of C major produces on its seventh degree and then largely ignores. Its notes — B, D, F — contain the tritone that makes the dominant work, and in practice the chord is nearly always replaced by G7, which contains all three plus a root underneath.
The notes
B, D, F — root, minor third, diminished fifth. Two stacked minor thirds, symmetrical and rootless in effect: any of the three notes can be heard as the bottom.
Add a G below and you have G7. That relationship explains why the diminished triad is so rarely played on its own: the same tension with a stronger foundation is available for one extra note.
How it is used
As a passing chord between two stable ones, usually with the bass moving by a semitone. On guitar it appears most often on three strings, since a full voicing adds nothing but difficulty.
It also functions as a rootless dominant: in a band context a guitarist playing B diminished over a G bass is playing G7, and the arrangement sounds fuller for the division of labour.
Where it shows up
As the vii of C major in theory exercises, and as a chromatic connector in practice — between C and Dm, or between two chords a tone apart.
The diminished seventh version, which adds a fourth note, is the more common form in jazz and in early twentieth-century popular song, where it appears as a matter of routine.