Heavy metal (often referre to simply as metal) is a genre of rock music that developed in the late 1960s and early 1970s. With roots in blues-rock and psychedelic rock, the bands that created heavy metal developed a thick, massive sound, characterized by highly amplified distortion and extended guitar solos. Allmusic states that "of all rock & roll's myriad forms, heavy metal is the most extreme in terms of volume, machismo, and theatricality."