Rock music is a genre of popular music that originated as "rock and roll" in the United States in the 1950s, and developed into a range of different styles in the 1960s and later, particularly in the United Kingdom and the United States. It has its roots in 1940s' and 1950s' rock and roll, itself heavily influenced by rhythm and blues and country music. Rock music also drew strongly on a number of other genres such as blues and folk, and incorporated influences from jazz, classical and other musical sources.Musically, rock has centered around the electric guitar, usually as part of a rock group with bass guitar and drums. Typically, rock is song-based music usually with a 4/4 time signature utilizing a verse-chorus form, but the genre has become extremely diverse and common musical characteristics are difficult to define. Like pop music, lyrics often stress romantic love but also address a wide variety of other themes that are frequently social or political in emphasis. The dominance of rock by white, male musicians has been seen as one of the key factors shaping the themes explored in rock music. Rock places a higher degree of emphasis on musicianship, live performance, and an ideology of authenticity than pop music.