As the 1960s progressed, Flatt and Scruggs broadened their sound and their audience.

The duo even performed at rock venues and festivals, as Scruggs wrote in "Earl Scruggs and the Five- String Banjo," an instructional book last revised in 2008

Source: http://www.bloomberg.com/news/2012-03-29/earl-scruggs-banjoist-who-invented-scruggs-style-dies-at-88.html