A treenail, also trenail or trunnel, is a wooden 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. Locust is a favorite wood when making trunnels in shipbuilding due to its strength and rot resistance.
A method of firmly securing 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", a term for decay accelerated and concentrated around metal fasteners. Increased water content causes wood to expand, so that treenails gripped the planks tighter as they absorbed water. Similar wooden trenail fastenings were used as alternatives to metal spikes to secure rail-support "chairs" to wooden sleepers in early Victorian times.
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