If you used floppy disk storage with your C64, one of the first hacking skills you probably learned was how to double the storage capacity of your floppies. I was thrilled when I first learned that I could double the amount I could store on a 5-1/4“ floppy (as they were valuable commodities to me back then) by simply notching the opposite side of the disk.
There were a few ways you could do this - with a knife, a hole punch, or even a “disk notching tool” that made its way into the market. I simply used an X-Acto knife, as it was the least expensive for me.
To get my hand-cut notch properly aligned, I would put two floppies face-to-face and use the factory-made notch as a template and cut the notch on the floppy below. It wasn't fancy, but it worked.
I remember doing this to an entire box of disks one time, later having my mother a little miffed about all of the “little black squares” littering the desk and carpet in my room.