W.C. Handy's "St. Louis Blues" is one of the most frequently recorded songs written in the 20th century. Of all those recordings, one stands out: Bessie Smith's 1925 version, with Louis Armstrong (1900-1971) accompanying her on the cornet -- a collaboration of three great figures (composer, singer, instrumentalist) in a new kind of music called jazz. Though the meaning of "jazz" is obscure, originally the term almost certainly had to do with sex. The music, which originated in New Orleans early in the 20th century, brought together elements from ragtime, slave songs, and brass bands. One of the distinguishing elements of jazz was its fluidity: in live performances, the musicians would almost never play a song the same way twice but would improvise variations on its notes and words.

Blessed with composers and performers of genius -- Jelly Roll Morton (1885-1941) and Duke Ellington (1899-1974), Louis Armstrong and Benny Goodman (1909-1986) and Bix Beiderbecke (1903-1931), Billie Holiday (1915-1959), and Ella Fitzgerald (1918-1996) -- jazz was the reigning popular American music from the 1920s through the 1940s. In the 1930s and 1940s the most popular form of jazz was "big-band swing," so called after large ensembles conducted by the likes of Glenn Miller (1909-1944) and William "Count" Basie (1904-1984). In the late 1940s a new, more cerebral form of mostly instrumental jazz, called be-bop, began to attract audiences. Its practitioners included trumpeter Dizzy Gillespie (1917-1993) and saxophonist Charlie Parker (1920-1955). Trumpeter Miles Davis (1926-1991) experimented with a wide range of musical influences, including classical music, which he incorporated into such compositions as "Sketches from Spain."