TREENAIL

Pronounced "trunnel" (rhymes with "tunnel"), a 'treenail', 'trenail', or 'trunnel' is a wood peg, or dowel used to fasten pieces of wood together, especially in timber frame construction and wooden shipbuilding. It is an ancient technology. Covered bridges in the U.S. often use treenails as fasteners. Many such bridges are still in use.
A method of making such a fastener was to cut a parallel peg of a softer wood, and then expand its outer end with a wedge of much harder wood driven into it.
Ancient Shipbuilding used treenails to bind the boat together. They had the advantage of not giving rise to "nail-sickness" which is a term for decay accelerated and concentrated around metal fasteners. Increased water content also causes wood to expand, so that treenails gripped the planks tighter as they absorbed water