This can help people who have trouble reading keep track of what line they're on. 120mm long and the gap to read from is about 5mm wide. I put a couple of bumps on it to help move it around the page, just in case someone has dexterity issues. Feel free to print and give to whoever you think might benefit from it.
This is a smaller sized one, easy to stash in your pocket or pencil case or something.

now included in the files is a 150mm model as well for bigger books.