Copland wrote El Salón México between 1932 and 1936, which met with a popular acclaim that contrasted the relative obscurity of most of his previous works. Inspiration for this work came from Copland's vivid recollection of visiting the "Salon Mexico" dancehall where he witnessed a more intimate view of Mexico's nightlife. Copland derived his melodic material for this piece freely from two collections of Mexican folk tunes, changing pitches and varying rhythms. The use of a folk tune with variations set in a symphonic context started a pattern he repeated in many of his most successful works right on through the 1940s. It also marked a shift in emphasis from a unified musical structure to the rhetorical effect the music might have on an audience and showed Copland refining a simplified, more accessible musical language.
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."
Copland's brand of nationalism in his ballets differed from that of European composers such as Béla Bartók, who tried to preserve the folk tones they used as close to the original as possible. Copland enhanced the tunes he used with contemporary rhythms, textures and structures. In what could seem contradictory, he used complex harmonies and rhythms to simplify folk melodies and make them more accessible and familiar to his listeners. Except for the Shaker tune in Appalachian Spring, Copland often syncopates traditional melodies, changes their metric patterns and note values. In Billy the Kid, he derives many of the work's sparse harmonies from the implied harmonic constructions of the cowboy tunes themselves.
Like Stravinsky, Copland mastered the ability to create a coherent, integrated composition from what was essentially a mosaic of divergent folk-based and original elements. In that sense, Copland's Populist works such as Billy the Kid, Rodeo, Appalachian Spring are not far removed from Stravinsky's ballet The Rite of Spring. Within that framework, however, Copland preserved the American atmosphere of these ballets through what musicologist Elliott Antokoletz calls "the conservative handling of open diatonic sonorities," which fosters "a pastoral quality" in the music. This is especially true in the opening of Appalachian Spring, where the harmonizations remain "transparent and bare, suggested by the melodic disposition of the Shaker tune." Variations which contrast to this tune in rhythm, key, texture and dynamics, fit within Copland's compositional practice of juxtaposing structural blocks