Introduction

Have you seen a parallel ruler? A parallel ruler is a navigational tool used to plot ship routes on charts. It is made up of two rulers connected with congruent crossbars. How is it used for navigation? One ruler is placed on a line connecting the ship's position to its destination point. The other ruler is moved onto a chart's compass to find the angle of the route. See the figure below.

Notice that portions of the rulers and the crossbars form a parallelogram.