Professional DJ CD Players are provided with a variety of useful features. They are categorized according to their type of construction: CD players that are designed like a turntable are called tabletop CD players. Compact players that can be built into a rack or that are installed permanently are called 19 inch players. Besides single players there are dual-players that are equipped with two drives in one housing. Players that are optimized to play music off of data storage mediums like USB sticks, SD cards or hard disks are called media players.