Details of a carefully wedged joint such as a cabinetmaker would make.
Popular in woodworking text books, but how often is it necessary or even advisable in real life? Consider that unless the joint is meticulously prepared and all goes to plan, you could find yourself with a badly fitting joint that you can't take apart.
The principal advantage is that once wedged, cramps can be moved and utilised elsewhere. Prior to wedging, to ensure a squarely set joint, you will need a cramp each side of the rail.
Snags:If you think that the splay increases the strength, take a look at the photos inDovetail Angles.
Some people like the look of the wedges, going as far a using wood of contrasting colour.
Note that unless the splayed tenon grips more firmly at its end, long term shrinkage of the mortised piece will cause the shoulder to open. The remedy is to ensure a firmer grip at the neck of the tenon, but then what's the point of having a splay in the first place.
Drive in the wedges at an equal rate or you'll find one sawcut firmly closed.