Portolan atlas of 9 charts and a world map The Library of Congress Geography and Map Division

       In the 13th and 14th century, Italian navigators developed the "portolano," a book that listed seaways, ports, and anchorages, as well as containing accurate and useful nautical maps. The English "portolan" is a shortened version of the word, which means "little port." Such charts were also called "rutters," from the French "routier," meaning "something that finds a way."

       Since the sea itself is featureless and sailors have no need of knowledge beyond the shore, portolan charts traditionally show only the blank ocean and its edge. Lines connected important destinations across the sea, while the land areas were left blank. Thus, on a portolan chart seaside cities look like stars, with lines radiating from them to show the compass headings for various destinations.

       The discovery of the magnetic compass in the 12th or 13th century revolutionized navigation. Mariners could confirm their course without relying on the sun or stars, since the needle always pointed in the same direction. (European compasses pointed north. Chinese compasses, developed around the same time, pointed south. It is not clear who came up with the idea first.)

       With a compass and a portolan chart, a navigator could leave port going in a specific direction, stay on that course, and arrive at land reasonably near his intended destination. Sailors could move out into the open ocean without having to follow along the shore or navigate from landmark to landmark, and could continue confidently on course even in cloudy weather. Thus, compass-and-chart navigation made it possible to "jump" from one city to another across the featureless sea.