Rather, the flat, circular magnetic disks that contained data were encapsulated within a hard plastic protective shell, adding rigidity to the format. Likewise, several niche optical disc formats from back in the day were protected by an outer shell, including the cult classic MiniDisc (MD) and the adorably miniature Universal Media Disc (UMD). With this in mind, it's peculiar that every mainstream optical disc format to have ever hit the scene has been designed without a protective shell to call its own. In a perfect world, I'd have loved to have seen CDs and DVDs ship with a built-in protective layer. If nothing else, I'm just glad that the market eventually sidestepped the issue by the time Blu-ray came around with its innovative scratch-resistant optical disc coating.