First mentioned on the year 948 in the Dutch city of Muiden, early cogs used a steering oar, had open hulls and could be rowed short distances. During the 13th century, cogs became larger and received decks and raised platforms in the bow and stern. Rudder cogs only appeared arround 1240.
Cogs also had an one yard sail, clinker outer planking at the hull's sides, straight stem and sternpost (as opposed to the rounded Viking ones), and strong cross-beams usually protruded through the ship's sides, holding them together.
During the 14th century some cogs were constructed with an oak hull, but clinker still dominated. By 1400 the cog had changed so much that it developed into a new type, the holk.