Oh, nice.

I used to make up RPG rules for fun and one of my systems used a die scale that went: d20, d40, d60, d80, d100, d120.

Those are just die sizes that are easy to make with the commonly available dice: a d20 is a single die, a d40 is a d4 and a d10 (reading units on the d4 and tens on the d10) and so on.

The thing is that d[20,40,60,80,100,120] is a nice, smooth scale, unlike what you get if you use the d[4,6,8,10,12,20] on their own.

And that works alright, but it's always nice to have the exact sizes.

"What do you do with 120-sided dice" indeed! Roll, them, duh. What else?