The Rainbows embellished the phrase as "do wop de wadda" in their 1955 "Mary Lee" (on Red Robin Records; also a Washington, D.C. regional hit on Pilgrim 703); and in their 1956 national hit, "In the Still of the Night," The Five Satins sang across the bridge with a plaintive "doo-wop, doo-wah." The vocal harmony group tradition that developed in the United States post-World War II was the most popular form of rhythm and blues music among black teenagers, especially those living in the large urban centers of the eastern coast, in Chicago, and in Detroit. Among the ﬁrst groups to perform songs in the vocal harmony group tradition were The Orioles, The Five Keys, and The Spaniels; they specialized in romantic ballads that appealed to the sexual fantasies of teenagers in the late 1940s and early 1950s. 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. They rehearsed on street corners and apartment stoops, as well as under bridges, in high school washrooms, and in hallways and other places with echoes: these were the only spaces with suitable acoustics available to them. Thus they developed a form of group harmony based in the harmonies and emotive phrasing of black spirituals and gospel music. Doo-wop music allowed these youths not only a means of entertaining themselves and others, but also a way of expressing their values and worldviews in a repressive white-dominated society, often through the use of innuendo and hidden messages in the lyrics. Particularly productive doo-wop groups were formed by young Italian-American men who, like their black counterparts, lived in rough neighborhoods (e.g., the Bronx and Brooklyn), learned their basic musical craft singing in church, and would gain experience in the new style by singing on street corners.