Parallel rulers

Parallel rulers are a bridge plotting instrument of two straight edges linked by hinged arms that keep them parallel as they are walked across the chart. The navigator aligns one edge with a charted course or bearing, then steps the rulers to the nearest compass rose to read the direction, or steps a known direction from the rose to the plotting position. Rolling parallel rulers achieve the same with a single ruler on rollers. They serve the same role as the course protractor, Douglas protractor, or station pointer, transferring directions between the compass rose and the working area of a paper chart.