Take a look at this picture: See the tape that is marking the ruler? It's translucent plastic and clings to the ruler without adhesive (kind of like the old Color Forms toys). I had a roll of it that I used it for years to mark cutting lines and alignment points and such on my rotary rulers. Now that I have used all of it, I can't remember where I bought the stuff or what it is called. I can't even remember if it came from the fabric store or the office supply. What it is, I haven't a clue. I don't even know if it was originally meant to mark rotary rulers, but it worked perfectly for the job.
Does anyone know what this stuff is and where to buy it?