Flippy disk
A single-sided floppy that users flipped over to record on the back — a cheap route to double the capacity.
Early 5.25-inch floppies were certified on one side, but the media itself was magnetic on both. Punching an extra write-protect notch and a second index hole on the opposite edge let you flip the disk and use the back in a single-sided drive. Pre-cut "flippy disks" shipped with notches on both edges; hobbyists managed with a dedicated hole punch.
The catch was that the drive read and wrote on the lower surface, so the flip side spun the "wrong" way and picked up wear from the pressure pad. Once double-sided drives became standard in the early 1980s, the trick faded.