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 very thin lines. The line width can be adjusted by an adjustment screw connecting the jaws. The adjustment screw can optionally have a number dial.
Illustration of ruling pen use from A Textbook on Ornamental Design (1901)