Rule of thumb for mortise-and-tenon size

Is there a rule of thumb for sizing a mortise-and-tenon joint?
—Michael Halloran, Richmond, Ky.

Of course, the specific mortise-and-tenon size will vary by application, Michael. But as a general guideline, make the tenon about ¹⁄₃ the total thickness of the tenoned workpiece, and about ¹⁄₂ –²⁄₃ as long as the width of the mortised workpiece. Though it's not structurally necessary, cutting a shoulder adjacent to the rail's edge as well as the face allows you to completely hide the mortise. To save setup time, size the edge shoulder the same as the face shoulder.

So, for example, for a matching ³⁄₄ ×2" stile and rail, first make the stile's mortise 1¹⁄₂  " long, ¹⁄₄ " wide, and 1"–1³⁄₈  " deep; then make a tenon sized for a snug fit without forcing, leaving it about ¹⁄₁₆ " short of the mortise depth to allow for glue.