The genre of rock music encompasses many distinct styles under the same umbrella. While the genre began with guitar - and piano-driven popular songs, today there are literally hundreds of variations on the original. Electronic rock contains elements of computer-generated or synthesized instruments, including drum machines and electronic guitars, in addition to some of the elements from the original movement. Heavy metal focuses less on melody and more on heavy guitars, while folk-rock typically uses a much more acoustic sound with instruments like banjos and harmonicas. Emerging in the late 1970s were glam rock and punk rock, which share a stripped-down sound and emphasize aesthetics, while punk rock tends to be more aggressive and glam tends to be more theatrical. Even today, rock music tends to share a distinctive beat with electric or acoustic guitars.