A drawing instrument commonly used to draw parallel lines is the?

The triangle is commonly used to draw parallel lines due to its straight edges and specific geometric properties. When placed against a straightedge or across the paper, a triangle can create perfectly parallel lines, especially when utilizing the right triangle form. This makes it an invaluable tool in technical drawing, architecture, and engineering designs where precision is crucial.

While a protractor is designed for measuring angles, a compass is primarily used for drawing arcs and circles, and a rule is generally used for straight-line measurements without inherently providing a way to maintain parallelism. The triangle specifically facilitates the drawing of parallel lines, demonstrating its unique effectiveness for this task.