The keel, or keelplank, was only slightly thicker than the adjacent garboards and had no rabbet. Both stem and stern-posts were straight and rather long, and connected to the keelplank through intermediate pieces called . The lower plank terminated in rabbets in the hooks and posts, but upper hoods were nailed to the exterior faces of the posts. Caulking was generally tarred moss that was inserted into curved grooves, covered with wooden laths, and secured by metal staples called "sintels". The cog-built structure would be completed with a stern-mounted, hanging, central rudder on a heavy stern-post, which was a uniquely northern development. The single, thick, mast was set forward of amidships, stepped into the keelplank and equipped with a single large, rectangular, square-rigged sail. The masts of larger vessels would be of composite construction. Complicated systems of rigging were developed to support the mast and to operate the sail. Cordage was usually hemp or flax and the sail hemp-based canvas. From the 13th century cogs would be decked, and larger vessels would be fitted with a stern castle, to afford more cargo space by keeping the crew and tiller up, out of the way; and to give the helmsman a better view.