Parallel rulers :
Used to plot direction on a nautical chart.
To determine the direction between two positions , line up the rulers with the two points and then step the rulers to the nearest compass rose printed on a chart, where you'll could read the azimuth.
This is a typical British method : Over here in France, to plot routes on ships and know what direction to tell their helmsmanto aim for, people use the Règle Cras ("Cras ruler", invented by Rear Admiral Jean Cras) which a compass rose printed on the ruler.
Brass rolling ruler
Ebony parallel ruler with brass hinges