Known as a mistar, it is a tool for marking lines on a page for calligraphy without drawing onto the paper. Heavy-duty thread runs in parallel lines, and the lines are transferred to the page by rubbing your finger over it. The lines disappear when the piece is mounted on board.
The mistar has a grid for perfect alignment and is marked both for a kitaa and with 4 full lines.
Master calligrapher Mohamed Zakariya can be seen demonstrating the use of the mistar in this video.