When it comes to defining the blues, there are several schools of thought, including the slick licks of Chicago, the soulful sounds of the Delta or the salty grooves laid down in Austin. These days, the blues are almost inescapable–it permeates rock (Led Zeppelin), folk (Ani DiFranco), jazz (John Scofield) and jam bands (Widespread Panic), as well as helping to launch all sorts of hyphenated strains. Back in the late ’40s, when Muddy Waters put his first blues band together, he set into motion a sound and style that would soon revolutionize popular music, leading to a tremendous blues movement that forged the way toward the formation of rock & roll. “The blues had a baby,” Waters once sang, “and they called it rock & roll!”