String Thang by Mark Wood is based on the twelve-bar blues. The blues has deep roots in American history, developed out of the work songs and chants of African-American slaves. The Great Migration of the first half of the 20th century saw freed slaves and their descendants move to northern and eastern parts of the U.S., taking their music with them. The blues took on different forms; in turn it helped to create rock, jazz, hip-hop, and other popular genres. A twelve-bar blues uses a repeating chord progression which allows musicians to express themselves creatively with each recurrence. If anything defines the blues, it is the use of blue notes"---pitches that don't usually belong in a given key---and elements of improvisation."