A ruling pen is a drawing instrument for drawing with ink or with other drawing fluids.
A ruling pen contains ink in a slot between two flexible metal jaws, which are tapered to a point. It enables precise rendering of the thinnest lines. The line width can be adjusted by an adjustment screw connecting the jaws.
Originally used for technical drawings in engineering and cartography together with straight rulers and French curves, it is today used for specific uses, such as picture framing or calligraphy.