What is "ruling pen"

Pen \Pen\, n. [OE. penne, OF. penne, pene, F. penne, fr. L. penna.]

1. A feather. [Obs.]
   --Spenser.

2. A wing. [Obs.]
   --Milton.

3. An instrument used for writing with ink, formerly made of a reed, or of the quill of a goose or other bird, but now also of other materials, as of steel, gold, etc. Also, originally, a stylus or other instrument for scratching or graving.

   Graven with an iron pen and lead in the rock.
   --Job xix. 2

4. 4. Fig.: A writer, or his style; as, he has a sharp pen. ``Those learned pens.''
   --Fuller.

5. (Zo["o]l.) The internal shell of a squid.

6. [Etymol. uncertain.] (Zo["o]l.) A female swan. [Prov. Eng.]

   Bow pen. See Bow-pen.

   Dotting pen, a pen for drawing dotted lines.

   Drawing pen, or Ruling pen, a pen for ruling lines having a pair of blades between which the ink is contained.

   Fountain pen, Geometric pen. See under Fountain, and Geometric.

   Music pen, a pen having five points for drawing the five lines of the staff.

   Pen and ink, or pen-and-ink, executed or done with a pen and ink; as, a pen and ink sketch.

   Pen feather. A pin feather. [Obs.]

   Pen name. See under Name.

   Sea pen (Zo["o]l.), a pennatula. [Usually written sea-pen.]

n. a pen used in graphic design and calligraphy for drawing precise lines of various thicknesses

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. The adjustment screw can optionally have a number dial.

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.