Proto-prog

[5] Some of the artists that were essential to the development of progressive rock, rather than just anticipating the movement, include the Beatles, the Beach Boys, the Doors, The Mothers of Invention, the Pretty Things, the Zombies, the Byrds, the Grateful Dead and Pink Floyd.

[8] Music writer Doyle Greene believes that the "proto-prog" label can stretch to "the later Beatles and Frank Zappa", Pink Floyd, Soft Machine, and United States of America.

[2] Conversely, academics Paul Hegarty and Martin Halliwell identify the Beatles, the Beach Boys, the Doors, the Pretty Things, the Zombies, the Byrds, the Grateful Dead and Pink Floyd "not merely as precursors of prog but as essential developments of progressiveness in its early days".

[9] According to Macan, the album may be the most influential to progressive rock for crystallizing the music of earlier "proto-progressive bands ... into a distinctive, immediately recognizable style".

[10] He distinguishes 1970s "classic" prog from late 1960s proto-prog by the conscious rejection of psychedelic rock elements, which proto-progressive bands continued to support.