Waylon Jennings/Willie Nelson

Waylon Jennings and Willie Nelson recorded together occasionally, most notably on "Mammas Don't Let Your Babies Grow Up To Be Cowboys." That song, released in 1978, became one of their best-known collaborations. They were both central figures in what was called outlaw country, a style that pushed back against the smoother production common in Nashville at the time.

Jennings had albums like "Dreaming My Dreams" and "Are You Ready for the Country?" in the mid-1970s. Nelson released "Shotgun Willie" and "Red Headed Stranger" around the same period. Their individual work often shared a similar, rougher-edged sound that distinguished them from their contemporaries.

They each had their own bands, Jennings with The Waylors and Nelson with The Family, but they played on each other's records. Their partnership wasn't a formal group so much as a recurring collaboration between two singers who had found a similar musical direction.