This thing is rad. I was in a fabric store a few years ago and came across a big display of quilter's squares. They are used to trim and square-up quilt blocks. Here's a nice video on how they are typically used. But I use mine to measure out panel borders when I'm penciling.
Quilter's squares come in various sizes and ratios but the one I use is 12.5 inches x 12.5 inches. It has marks every 1/8 inch. It's made out of plexiglass about an 1/8 inch thick. It's kind of heavy for a ruler, very rigid and it's transparent.
When I layout my panel borders with a regular ruler it's a process of measuring one side of the page, making a tic mark, then measuring the other side of the page making another tic mark and finally lining up my ruler between the two tic marks and drawing a line. With the quilters square it's just a matter of lining up the square and drawing a line.
I don't use it for inking panel borders because it's a little too thick to comfortably run my paint pen against. And it doesn't have that little cork riser that prevents the ink from wicking under the ruler.
So that's my quilter's square. I guess I only use it to pencil panel borders but it works great at that task.