A simple thing cut out of thin plywood. A point on one end and a couple saw-cuts on the other to mark optimal/minimum size for a tenon and dovetail.
The half-circle lets me get this in place when I have the tailstock engaged.
The picture above shows dovetail sizing, which is marked wide enough that the jaws will go into the recess. Below, you can see that the tenon mark matches the bottom of the jaws (widest part of the tenon).
I use these guides to either mark where to cut a tenon/dovetail, or to check (and adjust) the size after I’ve cut one by eye.