Blues-rock is a hybrid musical genre combining elements of the blues with rock and roll, with an emphasis on the electric guitar. It began to develop as a particular style in the mid-1960s in England and the United States through the work of bands such as Cream and The Rolling Stones, who experimented with music from the older American bluesmen like Elmore James, Bo Diddley, Robert Johnson, Howlin' Wolf and Muddy Waters. Blues-rock can be characterized by bluesy improvisation, the 12-bar blues, extended boogie jams typically focused on the electric guitar player, and often a heavier, riff-oriented sound and feel to the songs than might be found in traditional Chicago-style blues. It is also often played at a fast tempo, again distinguishing it from the blues.