Thrash metal (sometimes referred to simply as thrash) is a subgenre of heavy metal music, one of the extreme metal subgenres that is characterised by high speed riffing and aggression. Thrash metal songs typically use fast, percussive and low-register guitar riffs, overlaid with shredding-style lead work. Thrash metal lyrics also often deal with social issues using visceral and blunt language, an approach which partially overlaps with the hardcore genre. A lot of times the vocals are clean cut. The "Big Four" of American thrash metal are Slayer, Megadeth, Metallica, and Anthrax, who simultaneously created and popularized the genre in the early 1980s.