Before they became a saccharine pop band in the 1970s, Fleetwood Mac was part of the British blues explosion in the late 1960s, led by guitarist Peter Green, who disappeared by the early 1970s into drug-induced mental illness for decades, before emerging again in the new century.  This instrumental number was a #1 hit in Britain–indeed, in 1969, Fleetwood Mac was probably the top act in the U.K. in terms of major hits (including "Man of the World" and "Oh Well"), but none charted in the U.S.  This marks a good transition from the acoustic rock instrumentals to the electric ones.