The 5¼" floppy disks consisted of the floppy disk coated in magnetic substrate, encased a plastic envelope. The drive mechanism would only have one read/write head, to read one side of the disk. Disk manufacturers would sell single-sided floppy disks, as well as double-sided floppy disks that you could physically flip over to store more data on the other side. The double-sided floppy disks were a lot more expensive. The only real difference between the two types, though, was that the manufacturers warrantied that the second side would work; to save production costs, the disks were otherwise mostly the same.

The drives had a simple, mechanical write-protect sensor. If the edges of the plastic envelope were intact, putting a disk in the drive would block the sensor, and the drive wouldn’t allow writes. But, if there were a small notch cut in the edge, aligned with the sensor, the disk would not trigger the write-protect mode, and you could write to the disk.

The single-sided disks had a notch cut in one edge of the envelope to allow writing to one side of the disk. But, if you cut a notch in the same spot on the opposite side of the envelope, you could disable write-protect mode on the flip side of the disk. A hole punch was the easiest way to make the notch. Voilà! You could store twice as much data on the same disk.