The 18-inch (45.72cm) Aluminum Arms Parallel Rules feature aluminum arms that join two clear acrylic rulers enscribed with a protractor scale. An essential item in any proper navigation kit, these rules are used to plot direction on a nautical chart from any convenient meridian or compass rose. 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