Scarf joint with a wedge, in wood.
I have seen this joint in old wooden barns and houses in Britain on TV, and always wondered how and why it was done. Was it difficult to do? A random youtube video showed me how. While I do not build houses, from an engineering viewpoint it is educational to understand why it is so strong. No sources state the origins of this joint.
The scarf joint is elegant and also strong. The joint can be planed down flush. There is a lot of long grain glue surface area.