The Ruma bookmark ruler from Kaco has three functions: it will keep your place in your book or notebook, it will serve to measure lengths up to 15cm and you can also use it to draw lines. Many of us have probably done this anyway - kept an ordinary ruler handy within the pages of a notebook, to do just those things - so Kaco just made it a little more user-friendly.
The corners are rounded, for a start, so there's less chance of damaging the pages of the notebook. There's a handy clip, so there's no chance of the Ruma slipping out and getting lost, and it also makes it easy to pick it up from a flat surface. It's made of aluminium, so it's robust but lightweight, and it's available in basic colours to suit most notebook colours. This one's pale gold, with the centimetres and millimetres marked in black.