A good way to fasten tenons is to wedge them. This can be done whether the tenon goes through the mortise piece or only part way, as in a blind mortise and tenon joint.
The wedges can be driven between the tenon and the ends of the mortise, or, as is often better, driven into cuts made in the tenon itself, thus spreading the tenon towards the end, dovetail fashion, making it extremely difficult, or impossible, to pull it out of the mortise.
To wedge a tenon, one or more saw cuts should be made in it and carried farther than the wedges will extend. A small tenon can be split carefully with a chisel. Before wedging, the ends of the mortise should be tapered toward the side on which the tenon comes through. The mortise is tapered to match the angle of each wedge. The tenon and mortise having been glued, and glue also put into the saw cuts with a thin slip of wood, the tenon is fitted in place, and the wedges, previously prepared and tapering quite gradually, are dipped in the glue and driven down into saw cuts. Thus the end of the tenon is spread into a dovetail until it fills the mortise tightly, making a very hard or impossible to pull it out.
Wedged through tenon and mortise joint with a strain relief hole
Blind (Fox) wedged tenon and mortise woodworking joint
Blind wedging is sometimes resorted to when the mortise does not extend through the piece. The wedges must not be so long as to break off or to interfere with the tenon being driven home, and the bottom of the mortise should be quite smooth where the wedges strike it, or the wedges may turn to one side and not enter the tenon. The mortise is enlarged at the bottom. When everything is ready, apply the glue, start the wedges in the cracks, and drive the tenon to place. This will push in the wedges, spread the tenon at the end and fix it firmly. If well done, it cannot be withdrawn.