A double sider was a device that made a neat square hole in 3 1/2" and 5 1/4" floppy disks. It resembled a paper punch but instead of making a round hole, it made a square hole. The purpose was to turn so-called single-sided floppy disks into double-sided floppy disks.

In my product crimping node I've gone into some greater detail about the single sided versus double sided farce floppy disk manufacturers tried to get away with for a few years. In sum, people quickly realized that the only difference between the single- and double-sided floppy was the double sided had two holes (that is two read/write protect holes on opposite edges). If you punched a matching hole in a disk labeled as single sided, you double sided it. At first people would use a round paper punch to double side their 5 1/4" floppies. To make sure one carved out the right area, people tended to over punch, making it appear their floppy had been nibbled away at by some sort of mouse.

The 3 1/2" in a hard plastic shell was a tougher nut. Many people would drill a hole in the plastic. While that usually worked, sometimes some plastic dust from a sloppy hole drilling would work its way into case and gunk up the disk. Heating a nail up on a range top seemed to do a better job, although it produced a rather unpleasant smell.

It wasn't long, however, before manufacturers like Suncom came out with task-specific double siding devices that would make the proper sized hole. There was usually some kind spot where you could slot the disk in to ensure you punched the hole in the correct location. You could pick one up at any computer shop for about $20.