The mid-1960s has figured as a central period in the historiography of popular music, but the role of improvisation has been little discussed. This article argues that issues of improvisation and value are crucial to understanding the emergence of a high-low split within popular music, a division that figures prominently in criticism and fan discourse up to the present day. This new stratification within popular music made it possible for rock to acquire critical prestige relative to other popular music genres. The formation of rock also relied on its association with a primarily white, male, middle-class demographic. This article demonstrates that rock's prestige rests simultaneously on maintaining this narrow demographic profile while locating aesthetic and spiritual value in musical practices coming from elsewhere (in terms of geography, race, or cultural hierarchy): blues, Indian classical music, jazz. The socio-musical transformation in which improvisation played such an important role is explored through a survey of recordings and an analysis of the development of rock criticism in 1966, the year in which a new constellation of aesthetics, politics, and musical style crystallized.