The origin of the cog
First we have to ask ourselves if the cog, well-known in de Hanseatic area since the thirteenth century, did derive from a ship that was denoted as such in the ninth and tenth century. Although we do not have any finds of Early Medieval forerunners at our disposal, we have to presume that there must have been common characteristics with the Hanseatic cog, merely based on the name. A fast development, caused by the expanding trade in bulk goods, was the reason that little remained of the original nature of the small Frisian waddenschip, but the name cog sustained. The Frisians must have adapted a ship, that was originally of continental origin, to the specific circumstances of the shallow Frisian tidal waters. They must have used flat-bottomed ships which could navigate up the creeks and sink to the bottom at low tide for loading the ship. With such a ship merchants could sail to Denmark, well accessible through the tidal basin behind the barrier islands, called Wadden Sea.
Nordic ships were less suitable to navigate in the tidal waters of Western Jylland. For this purpose the Danes could have built ships of the Frisian type. For exactly wrecks of the earliest known cogs, from the twelfth century, were discovered at Kollerup and Kolding on Jylland.
In the western part of the Limfjord a closely related type still can be found nowadays, but disappeared from the Dutch waters in the 19th century. It is the so called kaag or kåg, with a carvel built bottom and slanting sides. In the Nordic shipbuilding tradition ships with carvel built bottoms are completely lacking (2). Cog-like ships without a keel are certainly not a Nordic development and must rather be considered to be a Frisian equivalent of the knarr, the well-known Nordic cargo-ship.

Naval cog organization
There is a hiatus of several centuries between the moment that the Haithabu coins were minted and the age of the oldest excavated cogs. However from the period in between there are some historical sources on the subject. For we have an indirect clue of the occurrence of the cog in the ninth century. We can deduce this from the way in which the general military duty (expeditio) locally was organized in the High Middle Ages. This was arranged in a different way in the county of Holland (the coastal territory in the western part of the Netherlands) than in other places. Holland was divided into naval districts, which were called koggen (cogs). In each district the inhabitants had the obligation to provide one heerkogge (war-cog), sufficiently manned and equipped. However this duty was not introduced by the counts of Holland, because references to these regulations can be found before their appearance. Already in a list of properties of the Church of Utrecht, drawn up in the late ninth or early tenth century, a tribute was mentioned which is called cogsculd (14). That was also registered in two charters dated in the tenth century (15). We may assume that we are dealing in these cases with a tax related to the cog duty.
The phenomenon of naval districts is also well-known outside Frisia. For there is a remarkable resemblance between the cog-districts in Holland and similar areas in Scandinavia.
According to the Heimskringla, the chronicle of the kings of Norway, King Håkon Haraldsson ordered to divide the land in skipreiður (ship-districts), reaching inland from the sea 'as far as the salmon swims upon the rivers' (16). All over Scandinavia we can find those districts, they were divided into smaller units, named after rowers. Each unit had to deliver one rower. And this reference is just what we also find in Holland. Because here the cog-districts were divided into some 20 to 30 smaller units, which were called riemen (oars), each responsible for the contribution of one able-bodied man, or armed rower.