Years ago some of the record companies (Rhino was one) came out with CD singles on 3.5" disks. They came with the required adapter to play on a standard CD player. Today all CD/DVD players and computer drives have the recessed well in the tray so these adapters are no longer needed. Some vertically mounted drives have a problem with the smaller disks though (the laptop kind don't have the tray and the disk is snapped directly onto the spindle so this isn't an issue). Sony even used to make a diskman that was sized to play those 3.5" disks, though it could also play the bigger ones (sticking out of the player!). I think I still have a few of those Rhino adapters someplace. The 3.5" CD singles are no more, turned out it was cheaper to just use a full size disk and only put 15 minutes of stuff on it than to have a separate line to make the smaller disks. The smaller disks did make a comeback, for business card sized CD-ROMs and for camcorders.