A question that I'm sure has been asked many times before?

What constitutes prog rock, as opposed to say, classic rock?

OK, many of the prog rockers of the era jumped on a bandwagon and seemed to think that if you could stretch "Three Blind Mice" to seven minutes, you were "prog", and I always try to avoid those tracks in the shows.

But do they have any merit? Where does prog start and classic rock, or just rock, finish?