Rock music is a genre of popular music that entered the mainstream in the 1950s. It has its roots in 1940s and 1950s rock and roll, rhythm and blues, country music and also drew on folk music, jazz and classical music. The sound of rock often revolves around the electric guitar, a back beat laid down by a rhythm section of electric bass guitar, drums, and keyboard instruments such as Hammond organ, piano, or, since the 1970s, synthesizers. Along with the guitar or keyboards, saxophone and blues-style harmonica are sometimes used as soloing instruments. In its "purest form", it "has three chords, a strong, insistent back beat, and a catchy melody."
In the late 1960s and early 1970s, rock music developed different subgenres. When it was blended with folk music it created folk rock, with blues to create blues-rock and with jazz, to create jazz-rock fusion. In the 1970s, rock incorporated influences from soul, funk, and Latin music. Also in the 1970s, rock developed a number of subgenres, such as soft rock, glam rock, heavy metal, hard rock, progressive rock, and punk rock. Rock subgenres that emerged in the 1980s included new wave, hardcore punk and alternative rock. In the 1990s, rock subgenres included grunge, Britpop, indie rock, and nu metal. A group of musicians specializing in rock music is called a rock band or rock group. Many rock groups consist of an electric guitarist, lead singer, bass guitarist, and a drummer, forming a quartet. Some groups omit one or more of these roles or utilize a lead singer who plays an instrument while singing, sometimes forming a trio or duo; others include additional musicians such as one or two rhythm guitarists or a keyboardist. Rock bands from some genres, particularly those related to rock's foundations in rock and roll, include a saxophone. More rarely, groups also utilize bowed stringed instruments such as violins or cellos, and brass instruments such as trumpets or trombones.