My mother-in-law, a pianist, taught me a dim chord as being a standard triad with both the 3rd and 5th being flatted. Or - two stacked minor third intervals. The ‘only three’ chords, with various inversions, is very cool, especially on a guitar where you literally repeat the chord everytime you slide the fingering pattern up three frets. Lots of opportunities.
FDPer Seth Rosen (RIP) was big jump blues/swing guy in Cleveland and I got to sub in his band a few times. A fave progression of mine was common in many of those songs. A couple runs of a brisk one; flatted two-dim; minor two; five...finished with a chromatic I-III-IV-bV-V run? He could rip his mandolin all around that progression for days, and I’d never get tired of it.
When I was teaching myself how to play guitar, the Mel Bay book used the term "diminished chord" to describe what is actually a "fully diminished 7th chord." As mentioned above, only three exist in nature.
I love diminished, half diminished, and major seventh chords because they seem to open up my ears to melodic possibilities that are never there if left to my own devices of three-major-chord pentatonic wankery.
I remember Seth. He was a cool cat. Badass mando player too.
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