Although a unidirectional English "progressive" style emerged in the late 1960s, by 1967, progressive rock had come to constitute a diversity of loosely associated style codes. When the "progressive" label arrived, the music was dubbed "progressive pop" before it was called "progressive rock", with the term "progressive" referring to the wide range of attempts to break with standard pop music formula. Author 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. Edward Macan says that psychedelic bands like the Nice, the Moody Blues, and Pink Floyd represent a proto-progressive style and the first wave of English progressive rock. Hegarty and 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".
Both the Beach Boys' Pet Sounds (1966) and the Beatles' Sgt. Pepper's Lonely Hearts Club Band (1967), with their lyrical unity, extended structure, complexity, eclecticism, experimentalism, and influences derived from classical music forms, are largely viewed as beginnings in the progressive rock genre. Critics assumed King Crimson's album In the Court of the Crimson King (1969) to be the logical extension and development of late 1960s proto-progressive rock exemplified by the Moody Blues, Procol Harum, Pink Floyd, and the Beatles. 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". 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.