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 similar to a vinyl record on a DJ turntable. Many have additional features that are not present on turntables, such as looping, beat analysis, and adjusting tempo independently of pitch. Additionally, some can function as DJ controllers to control the playback of digital files in DJ software such as VirtualDJ and Serato.
Many pro audio companies such as Gemini, Denon DJ, Numark, Stanton, and Vestax produced DJ quality CD players. In 1993 Denon DJ 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. Pioneer DJ CDJs have since become widely regarded as the industry standard.