The cog is a Northern European ship type which is thought to have originated in the North Frisian tidal flats. The oldest can be traced to before the ninth century. The cog is a wide, flat-bottomed ship with a straight stem and prow and high sides. The keel was a solid timber on which the prow and stern were built. The flat bottom was carvel-built (the planks lay edge to edge), while the sides were clinker-built (the planks overlapped). Originally cogs had one mast with a square sail, in the latest examples two masts. The oldest cogs had rudders like Viking ships, but the later ones had stern rudders, that is, the rudder was placed in the stern. This made navigation easier and safer.
The cog replaced the Viking ship as Northern Europes war and trading ship of choice, and the merchants of the Hanseatic towns in particular knew how to exploit the large new ship type. The cog had a considerable load capacity for the transport of mass goods such as herring in barrels, grain, and timber, precisely those goods which the Hanseatics transported from the Baltic region to England and the rest of Western Europe. Using cogs, the Hanseatic merchants and skippers sailed, from around 1200, ummelandt, which is to say, north of Skagen. They thus created the medieval eras extensive maritime trade between the Baltic States and Scandinavia at one end and England and the Western and Southern European countries at the other. The cog was the backbone of this maritime trade and thus the foundation of the economic and cultural upswing which Northern Europe experienced in the Middle Ages, and which Northern European-North American culture builds upon to this day.