A cog is a type of ship that first appeared in the 10th century, and was widely used from around the 12th century on. Cogs were clinker-built, generally of oak. These vessels were fitted with a single mast and a square-rigged single sail. They were mostly associated with seagoing trade in north-west medieval Europe, especially the Hanseatic League. Typical seagoing cogs ranged from about 15 to 25 meters (49 to 82 ft) in length, with a beam of 5 to 8 meters (16 to 26 ft) and were 30–200 tons burthen. Cogs were rarely as large as 300 tons although a few were considerably larger, over 1,000 tons.
Although the name cog is recorded as early as the 9th century, the seagoing vessel of that name seems to have evolved on the Frisian coast during the 12th century. Cogs progressively replaced Viking-type ships in northern waters during the 13th century. Why this was the case is uncertain, but cogs could carry more cargo than knarr of a similar size. Their flat bottoms allowed them to settle on a level in harbour, making them easier to load and unload. Their high sides made them more difficult to board in a sea fight, which may have made them safer from pirates.