During the era of great sea exploration in the 15th and 16th centuries, sailing ships traversed vast, hazardous oceans and navigated dangerous coastlines. Pilots used various navigation techniques—including a book called a “rutter” (not the “rudder,” the ship’s steering device). This was a log of events kept by earlier voyagers who chronicled their encounters with previously unknown and difficult waters. By reading the sailing details in a rutter, captains could avoid hazards and make it through difficult waters.