The oldest map in the collection of the Library of Congress is something quite extraordinary: a portolan of the Mediterranean Sea dating from between 1320 and 1350. Created on vellum (sheep or goat’s skin), it’s an abstract chart of ports joined by what are known as “rhumbs,” multicolored lines that radiate from “secret circles” designed to help ships navigate. Portolans (the term derives from the Italian portolani, “of ports”) came into being at the same time Europe first adopted the mariner’s compass in the late thirteenth century. These sea charts demonstrate a fascinating use of text–rather than trying to accurately draw physical features and coastlines, the maps are composed of the names of ports, bays and headlands. This particular chart, which was likely created in Genoa, looks like an abstract scrawl of information, until you suddenly start to picture the coastline of Italy, Greece, the Levant, and North Africa among the names. (Click on the illustration above for a much higher resolution to explore.)

(Above: detail of Genoan portolan showing coastlines of Sardinia and Italy, including Rome.)

The oldest surviving portolan is the Carta Pisane, created in the late 13th century and now held by the Biblotheque Nationale in Paris. Subsequent versions of portolans became even more elaborate, laden with illustrations of prominent geographical features, portraits of tribal rulers, imaginary monsters, and gorgeously intricate compass roses. Arab cartographers, including the great Piri Reis, created their own portolans that were both practical and aesthetically pleasing. Surprising, most portolans weren’t actually used for navigation; rather, they adorned the walls of prominent merchants and city rulers, emblems of the new power lines of transportation and trade.

(Portolan by Visconte Maggiolo, a Genoan cartographer, 1541.)

As the Age of Exploration (and Exploitation) began, more accurate maps were increasingly in demand, and the portolan fell out of favor, replaced by representational projection maps complete with lines of latitude and longitude. Yet these old charts drafted in red and blue inks upon the skins of dead animals remain a remarkable example of the union of form and function, a practical tool that’s also a thing of beauty.