Originally Posted by boo boo

Days of Future Past is my candidate for being the first true progressive rock album.

Some point to the Procol Harum debut released a few months earlier which is a great album and an influence on prog but it doesn't really sound that proggy and is more like a fusion of R&B and baroque pop.

Days of Future Past on the other hand is the true bridge between psychedelia and prog. In terms of the concept, the themes, the atmospheric feel, the song structures, the arrangements and most importantly the use of symphonic and classical elements. You can hear the clear influence it had on King Crimson, Genesis, Yes and others.

While they never surpassed DOFP or ever showed as much experimental ambition again, they still put out some solid albums in the late 60s and early 70s and don't get as much credit and respect as they deserve.