 The nonsense string of syllables, "doo doo doo doo-wop", from which the name of the genre was later derived, is used repeatedly in the song "Just A Sittin' And A Rockin", recorded by the Delta Rhythm Boys in December 1945.By the mid-1950s, vocal harmony groups had transformed the smooth delivery of ballads into a performance style incorporating the nonsense phrase as vocalized by the bass singers, who provided rhythmic movement for "a cappella" songs.Teenagers who could not afford musical instruments formed groups that sang songs "a cappella", performing at high school dances and other social occasions.