As it's name clearly indicates, Proto-Prog refers to the earliest form of progressive rock or progressive rock in embryonary state. The Proto Prog bands normally were formed and released albums before prog rock had completely developed, but there are some rare Proto Prog bands from the early 70's as well, because the genre didn't expanded to all the continents simultaneously. The common elements in all these bands is that they developed one or more elements of prog, and even when not completely defined as part of the genre, they are an important stage in the evolution of progressive rock. Generally, Proto Prog bands are the direct link between psychedelic and progressive rock and for that reason the psychedelic components are present in the vast majority of them, but being that progressive rock was born from the blending of different genres, Proto-Prog is any band that combined some elements of progressive rock with other genres usually prior to 1970. Some of these bands evolved and turned into progressive, while others simply choose another path, but their importance and contribution in the formative period of prog can't be denied.
The Beatles-Doctor Robert (Revolver, 1966 UK)