Ok, first lets distinct between progressive and prog. Or progressive and progressive. Progressive means something is stepping ahead and original, call it innovative. A copy of this kind of music cannot be innovative, because it's not original anymore, but still it's gets the same tag, hence the differences between progressive Rock and Progressive Rock or Prog-Rock. In this case, the latter terms are invalid, but describe a specific sound, progressive rock has numerous subgenres. Go to
http://www.progarchives.com/ to read about it and listen to examples, really helpful site. Post usually means a specific sound that deviates from the original standard, but got there by numerous steps. Post-Rock, for example, didn't jump from rock to GY!BE by the latter's appearence, it slowly evolved from dreampop/shoegazer and some classical minimalism, read about it on wikipedia. Think about it as the first death metal, didn't that sound almost exactly like thrash? Well, halfway, some band or critic starts attaching a label, and there you go, a new genre. Although I admit that original Post-Rock and Art-Rock are sometimes hard to separate, most of the genre is fairly easily recognized as such. This is not the case with the Avant-Garde. The Avant Garde is French, and means the front, the soldiers that go in first. So you might want to know what the difference is with Progressive. Well, when progressive has found something new, it usually stays there and holds onto it for a few albums or longer. A good example is Hawkwind, a progressive rock band that invented something (some say together with Pink Floyd) namely Space Rock, which they still play to this day. Because of the many following bands they had, it's not called progressive Rock, but got it's own tag: Space Rock. While some bands really went into untrodden grass with the Space Rock, like Acid Mothers Temple, which can therefore be called progressive, most bands playing Space Rock are not progressive, because they are copying something. So Hawkwind is progressive, and they still play what they invented over 30 years ago (more or less, I admit). The difference between Avant-Garde and progressive is now easy made, when you hear something comepletely new at an instance of time, it can be either, but when you hear something completely different every minute, you're dealing with the Avant-Garde. It's almost impossible to copy that style, because it can even be evolving within a song, taking new forms at all time, but generally around a project/artist/album fixed theme. Avant-Garde is often highly eclectic, incomprehendable and flatout weird. Avant-Garde has nothing to do with instumentation, style or form, it's not concrete, like innovation. Someone on this board once said that he thought of progressive as taking new elements in a prooved and tested basis, while avant-garde is "just way out there". That's very valid. I hope that helped.
