The photographs below show a treenail fixing that was recently made at Roskilde, Denmark. The plank is made of riven oak and the treenail made from carved willow. Note the direction of the grain on the head of the treenail. On the other side, you can see that the slit for the wedge was cut in line with the grain of the treenail. The wedge, from seasoned oak, has a radial grain which runs at right-angles to that of the willow. The tip of the wedge was hammered to two-thirds of the way through the hole in the oak block.
A treenail, also trenail, trennel, or trunnel, is a wooden peg, pin, or dowel used to fasten pieces of wood together, especially in timber frames, covered bridges, wooden shipbuilding and boat building. It is driven into a hole bored through two (or more) pieces of structural wood (mortise and tenon)
This short video shows a shipwright at Roskilde boring a hole through a plank.