I'm up early as usual, canoodling around with my guitar in D, strumming I, IV, vi, ii, etc., in no particular order, and then for some reason, when I played a Bb chord--not sure why I did that--it sounded great in with the others. Why would that be? It doesn't fit in the key, really, does it? Now as I type this, I've already forgotten the context: which chord I came from and which I went back to. I'll go try it again while you guys compose your answers.