A CDJ is a specialized digital music player for DJing. Originally designed to play music from compact discs, many CDJs can play digital music files stored on USB flash drives or SD cards. In typical use, at least two CDJs are plugged into a DJ mixer. CDJs have jog wheels and pitch faders that allow manipulation of the digital music file similar to a vinyl record on a DJ turntable. Many have additional features such as loops and beat analysis that are not present on turntables. Additionally, some can function as DJ controllers to control the playback of digital files in DJ software running on a laptop instead of playing the files on the CDJ.
Many pro audio companies such as Gemini Sound Products, Denon, Numark Industries, Stanton Magnetics, and Vestax produced DJ quality CD players. In 1993 Denon was the first to implement a 2 piece rack mounted dual-deck, variable-pitch, CD player with a jog wheel and instant cue button for DJs. It quickly became the industry standard and was widely adopted in most clubs and mobile DJs throughout the 90s up until 2004 when Pioneer made an impact with the CDJ-1000. Currently, Pioneer DJ CD players are now the most commonly found in dance clubs and are seen as the industry standard by many DJs.