 They often employed longer and more complicated compositions, interconnected songs as medley, and studio composition.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.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.