Why does a V7 sound natural to our western ears while a flatted fifth (which interval is part of a V7) sound so tense? I mean, I know the V7 *does* contain this tension and look for resolution, and I know what rich harmonic possibilities just this contains, however I am just wondering about this dramatic difference in the ear. If you are darkening something up by losing the root, why does it darken it up? I suppose a simplistic/mechanical answer is the having two tones (the 1 and 3 of the V7) from the major scale centers it on a given key whereare a two note b5 could be flip-flopped any which way - by its nature it is de-centered... Also is this why any given chord must have at least three tones (even if one of them is just an octave above)? There, at least there's a topic - I can't vouch for whether its a good one or not. Its interesting to me.