Nu metal (also known as nü-metal and aggro-metal) is a subgenre of alternative metal that combines elements of heavy metal music with elements of other music genres such as hip hop, alternative rock, funk, industrial, and grunge. Nu metal bands have drawn elements and influences from a variety of musical styles, including multiple genres of heavy metal. Nu metal rarely features guitar solos; the genre is heavily syncopated and based on guitar riffs. Many nu metal guitarists use seven-string guitars that are down-tuned to play a heavier sound. DJs are occasionally featured in nu metal to provide instrumentation such as sampling, turntable scratching and electronic backgrounds. Vocal styles in nu metal include singing, rapping, screaming and growling. Nu metal is one of the key genres of the new wave of American heavy metal. Nu metal became popular in the late 1990s with bands and artists such as Korn, Limp Bizkit, and Kid Rock all releasing albums that sold millions of copies. Nu metal's popularity continued during the early 2000s, with bands such as Papa Roach, Staind, and P.O.D. all selling multi-platinum albums, and came to a peak with Linkin Park's diamond-selling album Hybrid Theory. However, by the mid-2000s, the oversaturation of bands combined with the under-performance of a number of high-profile releases led to nu metal's decline, leading to the rise of metalcore and many nu metal bands disbanding or abandoning their established sound in favor of other genres. During the 2010s, there has been a minor nu metal revival; many bands that combine nu metal with other genres (for example, metalcore) emerged and some nu metal bands from the 1990s and early 2000s returned to the nu metal sound.