I'm putting up a split rail fence and am near the end of the project. I tried to plan the project to use full length rails for the majority of the fence run but I have about 5 locations where there must be shortened rails. The rails come as full 4x4's - they actually measure
4" x 4", and the ends have 2 tapers on each end that form the tenons. The oddball thing is that the tapers are cut across 2 faces so that when the tenon sits in the mortise, the rail is situated where you see 2 faces of the rail, as if it's sitting on one corner, and not a face- it's sort of discernable in the upper left and lower right rails of this picture:
Cheers