These are Corner Jigs for Routers.
You position them on a corner of some wood, clamp them and then use your Router with a flushcutter.
You might not be able to clamp them down. If so, use double sided tape. I designed them compact and short, because i don't need it clamped. Tape works great

The underside is slightly recessed, for you to mark it - if you want to.

Yo can also use them to sketch a roundedcorner or even turn them around and use them anywhere

If you're not sure wich size you might need, print 10mm and 20mm - those i find the most useful sizes in most of my woodworking projects.