Barbershop quartets, a type of music group fashionable in early twentieth-century America, had a dramatic influence on American popular music styles. The sweet, close harmony of the quartets, the arrangement of voice parts, and their improvisational nature were all influences in the development of doo-wop (already heavily improvisational in form) as well as pre-rock group singing, close-harmony rock groups of the 1950s and 1960s like the Beach Boys and the teenaged "girl groups," and in the later development of background groups and their vocal arrangements.

A barbershop quartet is any four-person vocal music group that performs a cappella, without instrumental accompaniment—the popular American music of the late nineteenth and early twentieth centuries. Each member of the quartet sings a particular voice part. One person in the group is considered the lead and sings the melody around which the other members base their harmonies. The tenor...