A keep case or poly-box is a type of packaging, most commonly used with DVDs and Blu-ray videos (and sometimes CDs). Besides DVD-Video films, keep cases are very common with most disc-based video games since the PlayStation 2, and they are also found on many PC titles and MP3-CD audiobooks (all use discs that are the same basic dimensions as a DVD). Products using keep cases The first products to be released in keep cases were VHS tapes, though most were stored in less expensive pressboard sleeves. In the days before video games were released on optical media, cartridges would rarely come packaged in specially designed plastic keep cases with lugs inside to hold the instruction manual, as opposed to pressboard boxes. Cartridges and cards for the Sega Master System platform were the first video games to be packaged in any kind of keep case. The vast majority of Sega Genesis/Mega Drive games would come in a plastic keep case (roughly the size of a VHS case), but they were later packaged in pressboard boxes as a cost-cutting measure. Before the standard and commonly used DVD case was invented, early CD-ROM-based video games such as Sega CD and early PlayStation titles would often come in a thicker and taller jewel case. These have been reputed to break very easily. Since the PlayStation 2, most major console-based video game software is packaged in some sort of keep case, including Nintendo DS cartridge titles and Sony PSP titles. Structure Materials and features The cases are made of soft, clear or colored, polypropylene plastic with a transparent polyethylene outer jacket, usually with a printed paper sleeve behind it. A variety of colors are produced, including black, white, red and clear. On the inside are sets of clips that may hold a booklet or additional sheets of paper with extra information. The DVD is held into place with a small protrusion (a "hub") which fits into the hole in the DVD. For console games like the PlayStation 2 and Nintendo GameCube, an extra protrusion is available for storing memory cards. Types of hubs The structural differences between a CD and a DVD have led many manufacturers to study different hub designs for keeping the DVD (or the DVDs, in multi-disc cases) in place: unlike CDs, which are made from one layer of plastic material, DVDs have two layers, which are thinner (so that together