Parallel rulers were first developed in the 16th century but not adopted by navigators much before the 1700s. They work with the compass rose on the chart to transfer any angle – for instance for a position line, course we have steered or course we want to steer.
Most also have the Captain WA Fields scale in degrees on the top rule and compass points on the lower which, when used in conjunction, on a meridian, make for easier chart plotting, without the need for “walking” the rules across the chart on a chart table or deck which is heaving about.
This video is not about navigating using three point fixes, estimated position plots or courses to steer. It mentions DR – dead reckoning in passing; it’s a basic refresher about the uses of parallel rules in chartwork.
Also remember that by using parallel rules you are effectively chipping the rust off your hippocampus… that’s the part of your brain that helps you to remember how to navigate!