(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.