I can add to the foregoing that...
Thinking about the scarf joint kanawa-tsugi a little further, I realize that it is a little hard to say whether the net effect of driving in the wedging pin is to put the joint in compression or tension. Obviously the end grain abutments are in compression. So that might not be such a good example to have brought forth.
I think the case is much clearer with the rod joint that the internal tension of the joint is increased as the wedges and/or peg are driven in. Also true for adapted forms of the joint, as seen in 2-, 3- and 4-way splined connections.
