Alternative metal (also known as alt-metal or hard alternative) is a subgenre of rock music and heavy metal with influences from its parent-genre alternative rock, and other genres not normally associated with metal. Alternative metal bands are often characterized by heavy guitar riffs, melodic vocals, sometimes harsh vocals, unconventional sounds within other heavy metal genres, unconventional song structures and sometimes experimental approaches to heavy music. The term has been in usage since the 1980s, although it came into prominence in the 1990s. It has spawned several subgenres, including rap metal and funk metal, which have both influenced another prominent subgenre, nu metal, which expands the alternative metal sound, commonly adding influences from hip hop, groove metal, grunge, and sometimes industrial metal.