"El Salón" prepared Copland to write the ballet score "Billy the Kid", which became, in Pollack's words, an "archetypical depiction of the legendary American West". Based on a Walter Noble Burns novel, with choreography by Eugene Loring, "Billy" was among the first to display an American music and dance vocabulary. Copland used six cowboy folk songs to provide period atmosphere and employed polyrhythm and polyharmony when not quoting these tunes literally to maintain the work's overall tone. In this way, Copland's music worked much in the same way as the murals of Thomas Hart Benton, in that it employed elements that could be grasped easily by a mass audience. The ballet premiered in New York in 1939, with Copland recalling: "I cannot remember another work of mine that was so unanimously received." Along with the ballet "Rodeo", "Billy the Kid" became, in the words of musicologist Elizabeth Crist, "the basis for Copland's reputation as a composer of Americana" and defines "an uncomplicated form of American nationalism".