Hmm ... Mark said a diminished chord can be named after any of the 4 notes it contains, because they are all equally spaced (all minor 3rds) but that's actually the characteristic of a diminished 7th chord. A "just plain" diminished chord has only 3 notes (all a minor 3rd apart) and it's named after the one that would be on the bottom if you stacked them in 3rds, same as with a major or minor chord. At least, that's the "music theory" nomenclature. I don't know if anyone ever actually plays just plain diminished (as opposed to diminished 7th) chords on a guitar, especially considering the dim7 is more generally useful ... JZG