Progressive bluegrass is one of two major subgenres of bluegrass music. It is also known as newgrass, a term attributed to New Grass Revival member Ebo Walker. Musicians and bands John Hartford, New Grass Revival, J.D. Crowe and the New South, The Dillards, Boone Creek, Country Gazette, and the Seldom Scene pioneered innovations in the genre. Some groups began using electric instruments and importing songs from other genres, particularly rock & roll. Progressive bluegrass became popular in the late 1960s and 1970s, but it can be traced back to the banjo and contrabass duets that Earl Scruggs played even in the earliest days of the Foggy Mountain Boys. The four key distinguishing elements of progressive bluegrass are (1) instrumentation, frequently including electric guitars, drums, piano, and more, (2) songs imported or styles imitated from other musical genres like jazz, rock and others, (3) non-traditional chord progressions, (4) lengthy "jam band"-style improvisation. However, not all these elements are always present in progressive bluegrass.