The cog was a small sailing vessel introduced in the 10th century. Cogs were generally built of oak, which happened to be abundant in the Baltic. This vessel was fitted with a single mast and a square-rigged single sail. Although this type of rigging prohibited sailing into the wind, it had the benefit of a smaller crew, which reduced operational costs. These vessels were mostly associated with seagoing trade in medieval Europe, mostly in and around the Baltic Sea region.

Cogs were built with flush fitting planks on the bottom and amidships. These planks gradually gathered and overlapped at the posts. Cogs also had a stern mounted rudder for better control. This differed from the side mounted rudders of most galleys at the time.

Cogs are first mention in 948 in Amsterdam. They are said to have been developed from the Norse, knarr. The knarr was a long shallow draft ship fitted with a single mast and sail. They did not use a stern rudder like the cog. Recent evidence looks to the Frisian coast, Western Jutland, as the possible birthplace of this type of vessel.
The cog with is flat bottom and shallow draft was well suited for sailing in the Baltic region. The transformation of the cog into a true seagoing trader came not only during the time of the increasing trade between West and East. It could also been a direct answer to the closure of the western entrance to the Limfjord. Limfjord in northern Jutland offered fairly protected passage between the North Sea and the Baltic. However the passage was constantly filling with sand and was completely blocked by the 12th century. Bigger ships that could not be pulled across the sand bars had to sail around the Jutland peninsula and circumnavigate the dangerous Cape Skagen to get to the Baltic. However the cog could cross the sand bars and reopened the Limfjord to trade traffic.

Cogs were even used as military ships. Forecastles and stern castles were fitted to the hulls. These location mounted crude cannon and offered prepared fighting positions. By the 14th century the cog had reached the limit of its design.