This was used to cut out a notch on the top corner of a 5 1/4″ floppy disk to let you format, write to, and use the other side. Floppy disk manufacturers would sell both Single-sided and Double-sided disks, charging a significant premium for the double-sided floppies. It was said that the double-sided disks had higher quality-control standards for the reverse side of the magnetic media as well as the front, but the reality was that the only significant difference between single-sided and double-sided disks was an extra notch in the reverse corner of the disk.
