I've been making my way through the Practical Music Theory PDF and am trying to memorise the (triad) chords in various keys of the major scale. The VII degree of each major key is a diminshed chord - but these are chords I do not know. I don't think I'm going to make the effort to learn them either - not at this point anyway. From what some of you have said above this seems sensible.
I know my Dominant Sevenths cos Justin taught me them - and I've learned to play some blues. I realise these chords are quadads rather than triads (thanks again J). Now I know there are also Minor Sevenths 1, 3b, 5, 7b), Major Sevenths (1, 3, 5, 7), Diminished Sevenths (1, 3b, 5b, 7bb) and Half-dimished Sevenths (1, 3b, 5b, 7b)!