knurl is a strange word.
@lizardsquid https://upload.wikimedia.org/wikipedia/commons/b/bf/Knurling_closeup.jpg here's what it looks like! The ridged pattern common on plastic knobs is also sometimes called knurling though it might not technically count, I dunno
@lizardsquid and I guess it's used on more than just rotatey things, it's good for grips on tools in general
@lizardsquid you've seen knurled things almost certainly though; it's where they put a rough texture on something to make it easy to turn, common on knobs and stuff