In what way did Chicago's music scene in the early to mid-20th century contribute to popular music such as jazz, R&B and rock'n'roll?

Like Detroit, Chicago benefitted immensely from the so-called Great Migration of African Americans in the early to mid-20th century from the South to the North, and particularly, the industrial mid-west. Jobs in manufacturing were plentiful in cities like Chicago, Detroit, Cleveland and Philadelphia, so African Americans looking for better jobs with better pay moved to those cities in the hundreds of thousands, from the start of the 20th century up until the 1950s. Those African Americans brought with them the so-called "Negro Spirituals," Afro-Caribbean-influenced spirituals hymns that slaves had sung for generations, whose rhythms and reliance on improvisation, as well as call and response, served as a backbone for Jazz, Rhythm & Blues and Rock. 

By the late 1890s and early 1900s, Chicago's South Side had become a culturally rich neighborhood compromised almost entirely of African Americans, and many who had migrated from the South, particularly New Orleans, brought their musical talents with them. Some played ragtime, which was a precursor to jazz. On the South Side, from 1910 and later, increasingly so during Prohibition, black cabaret musicians dominated the night club scene, providing musical entertainment to night clubs that served alcohol. Many black musicians from New Orleans came to Chicago in the 1920s because Chicago had plenty of recording studios, whereas New Orleans did not. Louis Armstrong came to Chicago to play, as did Miles Davis later on.

But even earlier, in 1906, some of the earliest jazz pianists such as Tony Jackson and "Jelly Roll" Morton came to play in Chicago. By the late 1920s, the large African American audience for jazz in Chicago had resulted in black music entrepreneurs opening specialty recording labels such as Okey, Paramount, and later, Chess Records. These labels came to be known as "race records," and were consumed primarily by African American audiences, before later coming to the attention of white musicians and music producers, who later "mainstreamed" these musical traditions. A great example of this is band leader Benny Goodman, who was white, and later became the "King of Swing."

Like Detroit and New York, Chicago was a major musical venue and cultural melting pot for jazz and blues musicians from around the country, who borrowed from and copied each other, inspiring some of the greatest innovations in popular music of the 20th century. The urbanization and commercialism of jazz music in post World War Two Chicago helped give rise to the genesis of Rock & Roll, and allowed African American recording artists to become mainstream musical superstars, whose fans were both black and white.