A treenail, also trenail or trunnel, is a wooden peg or dowel

Dowel

A dowel is a solid cylindrical rod, usually made of wood, plastic or metal. In its original manufactured form, dowel is called dowel rod.Dowel rod is employed in numerous, diverse applications. It is used to form axles in toys, as detents on gymnastics grips, as knitting needles, as structural...

 used to fasten pieces of wood together, especially in timber frame construction and wooden shipbuilding

Shipbuilding

Shipbuilding is the construction of ships and floating vessels. It normally takes place in a specialized facility known as a shipyard. Shipbuilders, also called shipwrights, follow a specialized occupation that traces its roots to before recorded history.Shipbuilding and ship repairs, both...

. It is an ancient technology. Covered bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

s in the U.S. often use treenails as fasteners. Many such bridges are still in use. Locust

Honey locust

The Honey locust, Gleditsia triacanthos, is a deciduous tree native to central North America. It is mostly found in the moist soil of river valleys ranging from southeastern South Dakota to New Orleans and central Texas, and as far east as eastern Massachusetts.-Description:Honey locusts, Gleditsia...

 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.