One of America's few indigenous musical forms, the blues developed from a polyglot of sources - the African "griot" or storyteller tradition brought here by black slaves, gospel music, work and prison songs that often used a "call and response" pattern, and the rhythms of ragtime piano music. As Alex Gibney puts it, this music born of slaves and sharecroppers reflects the American paradigm, "something that has individual power and strength but is able to allow for and integrate a number of other voices."

The founding voices of American blues, most of them born in Mississippi, include Robert Johnson (who, legend has it, struck a deal with the devil in exchange for his guitar skill), Charley Patton, Son House, Elmore James, Sonny Boy Williamson II, and Howlin' Wolf, bluesmen who wrung their art out of poverty, back-breaking physical work, and discrimination.

"This strange music started coming from the most oppressive, harsh conditions," says Charles Burnett, who was born in Vicksburg, Mississippi, and came home to film "Warming by the Devil's Fire," his contribution to the PBS series. "In spite of all that, these talented, expressive people created this poetry and this new American art form."

For decades, blues was almost exclusively Southern, African-American music. That began to change just before and during World War II, when many black musicians made their way north from the Delta along with thousands of other African-Americans, fleeing Jim Crow racism and seeking wartime employment. In the bigger, noisier cities like Chicago, musicians like Muddy Waters, Bo Diddley, and B.B. King shifted to electric guitars and began to attract a smattering of whites to their shows. AM radio also began to take the blues to a wider audience.