Product details:

Of all our traditional parallel rulers, the 15-inch (38.1cm) Brushed Aluminum Arm Parallel Rule is a best-seller. This sturdy acrylic parallel rule includes a protractor scale, is easy to use, and is a necessary tool for every seagoing navigator. The two straight-edged rulers are connected to maintain the same angle. By walking the rules across the chart and securely holding down the non-moving ruler, you can effectively plot a course.