Chicago blues is a form of blues music developed in Chicago, Illinois, in the 1950s, in which the basic instrumentation of Delta blues—acoustic guitar and harmonica—is augmented with electric guitar, amplified bass guitar, drums, piano, harmonica played with a microphone and an amplifier, and sometimes saxophone. The best-known Chicago blues musicians include singer-songwriters and bandleaders Muddy Waters, Howlin' Wolf, and Willie Dixon; guitar players such as Elmore James, Luther Allison, and Buddy Guy; and harp (blues slang for harmonica) players such as Little Walter, Paul Butterfield, and Charlie Musselwhite. Since the 1960s, the Chicago blues style and sound has spread around the US, the UK and beyond.