Viking ships were usually made of oak which had been split into planks, although pine was also used for decks and masts. The planks were fastened to a keel and then to one another with iron nails, which are visible here. This "clinker" technique gave them a shape which was different to that from ships built more conventionally by first building an inner skeleton for the hull. Next, the Vikings attached evenly spaced floor timbers to the keel and not to the hull, making the ships more resilient and flexible. Finally, they added crossbeams to make a deck and rowing benches, and secured a huge beam along the keel to support the mast. (Thanks to Evan Hadingham on www.pbs.org/wgbh/nova/ancient/viking-ships.html for this information.)