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Not as commonly seen ‘in the field’ as your more regularly featured 7ths, 9ths and 11ths and so on, the diminished 7th chord (Root- b 3-5-7; yes, that’s double 7!) is still an essential part of diatonic harmony – or the chord scale, if you prefer. We have a set structure for the major scale – tone-tone-semitonetone-tone-tone-semitone (TTSTTTS, for short) – there is a structure to how this is arranged in diatonic/chordal harmony: major-minorminor-major-major-minor-diminished (or m7 5)-major.