I like to keep a bullet journal in blank paged notebooks, but occasionally having college ruled lines or graph paper is quite handy. The stencil here acts as a bookmark and 10 cm ruler, but can be used to create lines and grids. 

The only major innovations here (if it can be called that) are 1) the bottom and top lines have viewing windows that allow for a user to orient the stencil very quickly on existing lines for rapidly creating grids and line columns larger than the stencil, and 2) the ruler lines extend across the stencil lines, allowing for a user to stop their lines at the same point (in case you don't want them to run the entire stencil).

To get the ruler, you obviously need two different colored filaments. I have included two versions - one for a simpler filament change at the .5 mm mark (labeled ‘Single Print), and another for an MMU or pseudo-MMU setup, where the ruler is printed on the bottom (yeo gilrd labeled ’MMU'). I prefer the second one, as it makes the whole stencil flat, which is nice on the notebook pages when used as a bookmark and generally feels more professional. That being said, it takes a little more effort.

A link to the tinkercad file (with the engraving and the stencil separate) is here for anyone who wants to play with it.