The traverse board marker was introduced with the dissemination of the magnetic compass to show the changes in a ship's course and speed. The instrument served to circumvent the problem of sailors' illiteracy, which prevented them from compiling the logbook or marking information on the sea chart. The device consisted of a wooden board displaying a windrose with 32 rhumb lines in bright colors. Each rhumb line carried eight holes, one for each half-hour of a sailor's watch. Every half hour, the helmsman inserted a wooden peg in the hole after the one marking the previous half hour, on the rhumb line matching the one shown on the compass. The pegs carried notches showing the speed in knots, so the positions of the pegs gave a precise indication of the ship's course. At the end of the watch, the navigator copied the navigation data onto the chart to plot the ship's position.