There are seven notes in a major or minor scale, but the basketful of chords that we gathered the other day included only six diatonic triads. What happened to that seventh note? Doesn't it get a chord too?
The answer is yes—and no. There is a chord built on the seventh note of the major scale, but it is very rarely used in rock songs. This forgotten chord is:
[seven]° or [seven]dim
in C major: B° or Bdim
This chord contains the notes 7, 2, and 4 — in C major, the notes B, D, and F. The interval from B to F is a diminished fifth, unlike the perfect fifth found in the other six diatonic triads. And while we love the resonance of the perfect fifth, we squirm at the awkward dissonance of the diminished fifth.
Probably a more important reason for this chord's neglect in rock is that it's just so hard to play on guitar. You have to delicately stop one string from sounding while playing the strings around it. Guitarists are so universally embarrassed about this difficulty that they don't even admit to the existence of the diminished chord. Guitar chord charts universally show the easy-to-play diminished seventh chord when a diminished chord is called for. The diminished seventh chord is an interesting chord in its own right, with a freakish (and useful) four-way symmetry, but it's not the same chord as a diminished chord.
In practice, the diminished chord, when it's used at all, is found in keyboard-oriented songs. The guitar player just sits out that part of the song.
In the minor key, the same points apply to the [two]dim chord. In the key of C minor, this is Ddim, containing the notes D, F, and A♭.
In music theory, the [seven]dim chord is thought of as a stunted version of the [five]7 chord. It's expected to be followed by the [one] chord most of the time.
Though this chord #7 turns out the be the neglected runt of the litter, there is another chord #7 that plays a major role in rock music. If you just lower the B note in Bdim, the note becomes B♭, and the chord becomes B ♭, or [seven]♭, the subtonic chord. This glorious major triad is so important in rock that it deserves its own entire post, which we'll get to soon. In the meantime, listen to the fadeout of "Hey Jude," and practice saying the cool Greek word mixolydian.