Rock music is a genre of popular music that entered the mainstream in the 1960s. 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, bass guitar, drums, and keyboard instruments such as hammond organ, piano, or, since the late 60s, synthesizers. Rock music typically uses simple unsyncopated rhythms in a 4/4 meter, with a repetitive snare drum back beat on beats two and four.[1] Guitar solos feature prominently in rock music, however keyboard, saxophone and blues-style harmonica are also sometimes used as soloing instruments. In its "purest form", it "has three chords, a strong, insistent back beat, and a catchy melody."[2]