Originally Posted by GravitySlips

this happens all the time in music, though. look at post-rock, krautrock, post-punk, etc etc... they aren't so much descriptions of a SOUND, they're more just umbrella terms for a musical ideology or aesthetic. Swell Maps and Joy Division hardly sound similar, but they're definitely both post punk bands.

Classic rock is a particularly useless grouping though, in my opinion. I certainly don't think of Black Sabbath as a 'classic rock' band by any stretch of the imagination.