may use a monitor speaker to hear the house's main mix. This monitor speaker can have its volume increased or decreased by the DJ as needed. Power DJ mixers have an AC mains plug that is connected to the wall to supply electric power for the unit. Some DJ mixers can take batteries, which enables users to mix songs outside or away from electric power sources, with the output being plugged into a portable boom box or other battery-powered sound system. History DJ mixing played a key role in the development of the African-American style of hip hop music. In hip hop music and occasionally in other genres that are influenced by hip hop (e.g., nu metal), the turntable is used as a musical instrument by DJs, who use turntables along with a DJ mixer to create unique rhythmic sounds and other sound effects. Manipulation of a record as part of the music, rather than for normal playback or mixing, is called turntablism. The basis of turntablism, and its best known technique, is scratching, pioneered by Grand Wizzard Theodore. It was not until Herbie Hancock's "Rockit" in 1983 that the turntablism movement was recognized in popular music outside of a hip hop context. In the 2010s, many hip hop DJs use DJ CD players or digital record emulator devices to create scratching sounds; nevertheless, some DJs still scratch with vinyl records. DJ mixing also played a key role in disco music in the 1970s. In disco clubs, DJs would use mixers to transition seamlessly from one song to another and create a mix of songs that would keep the dancers energized. One of the pioneers of DJ mixing equipment design was Rudy Bozak. Beatmatching and beatmixing with a DJ mixer were first used to encourage dancers not to leave the dancefloor between songs. By beatmatching song A and song B, a DJ can transition seamlessly between two songs, without creating a break in the music. Typically, when a DJ is using beatmatching, they will let song A play until it nears its end. As song A is playing, the DJ "cues up" song B in their headphones and adjusts the speed of record player B until the beats of song A and song B are synchronized ("synced up"). Then, once the two songs' beats are synchronized, they can slowly fade in song B while song A is nearing its ending.