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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{{dim}} chord''' (or vii or viio when the {{dim}} symbol cannot be made). The {{dim}} symbol indicates diminished, but you have to be careful - we are dealing with the diminished triad here, not the [[7th Chords#Half Diminished|diminished 7th]]. The diminished 7th chord has two variations - half and full. Half is indicated with a slash through the {{dim}} symbol (B{{hdim}}{{7}}). Full uses the normal symbol (B{{dim}}{{7}}).  <small>''Where the {{dim}} symbol cannot be made, such as on a standard keyboard without using special character codes, or to avoid ambiguity, you can use "dim" (ex. Bdim) to indicate the diminished quality of the chord. Then a half diminished 7th can be written as Bm7b5''.</small>

The vii chord is called the '''leading chord'''. Harmonically, it has a ''dominant'' function, sharing the leading tone and 2nd of the scale with the V chord(B and D in the key of C Major).

C F Bdim Em Am Dm G C I IV viio viio iii vi ii V I E--0-----1-----1-----0-----0-----1-----3-----0-- B--1-----1-----3-----0-----1-----3-----3-----1-- G--0-----2-----x-----0-----2-----2-----0-----0-- D--2-----3-----3-----2-----2-----0-----0-----2-- A--3-----------2-----2-----0-----------2-----3-- E--------------------0-----------------3--------

The vii{{dim}} chord is the only diatonic chord in any major key to contain the diminished 5th interval. Remember that the most harmonic interval (other than an octave) is the perfect fifth, represented by the simple ratio of 3:2. By contrast, the diminished 5th has a ratio is 745:532. Let's see all the intervals again:

| '''d5''' || '''745:532'''

| m7 || 169:95

As you notice, the intervals closest to the root note or octave feature the larger integer numbers for their ratios and have the largest least common multiples. As we move away from the root/octave, the ratios seem to get more and more harmonic. Yet, the d5 is a major departure from the general trend - it appears the least harmonious of all. (Even in an alternate tuning where its ratio is a more harmonic 7:5 variation, it is about as harmonious as the minor 3rd and 6th, far from the perfect 4th and 5th. ) As it serves as the replacement of the P5 in this chord, this immediately makes the chord quite dissonant.

It's quite apparent when playing the B{{dim}} chord. This means the vii{{dim}} chord not only has tension in the context of the tonality and its placement inside a progression, but it is also quite dissonant and tense in and of itself. Typically, we don't see diminished triads as frequently as [[7th Chords|7th]] or other [[Extended Chords|extended chords]]. Its half diminished variety (diatonic to the major, natural minor, and harmonic minor scales) variety actually adds consonance, which we will cover [[7th Chords#Half Diminished|later]].