A Breton plotter, also known as a Portland Course Plotter, is a navigational instrument used for nautical navigation with charts. It is a ruler with a rotating rose that enables to plot a course on charts, while aligning the North of the ruler with the North of the chart.
The Breton Plotter was invented by Captain Yvonnick Gueret, a Breton seaman who developed the plotter during his experiences teaching navigation. He was a commander in merchant vessels, served on small fishing boats, delivered private yachts and taught navigation. He later extended the use of the plotter to air navigation for small aircraft.
References
Breton plotter Wikipedia(Text) CC BY-SA