Mortise & Tenon Joints

By David J. Marks

The construction of furniture can be divided into two main categories; casegood and frame.

Casegood construction utilizes dovetails, dowels, mortise and tenons, and finger joints, to name a few.  Frame construction traditionally relies on mortise and tenon joints.  This is a joint that is very strong and has a positive registration.  It is probably the oldest and most widely used joint in woodworking.  Before the advent of modern adhesives, mortise and tenon joints were locked in place with pegs or wedges.  Frame construction is the method usually used to build doors (frame and panel), chairs, tables, chests, etc.  The general guidelines when cutting these joints are that the tenon should be approximately one third the thickness of the rail.  I’m referring to the construction of a flat frame which consists of rails and stiles such as a typical door frame.  Now when we talk about post and rail construction, for example a table leg and apron, the rail is usually thinner than the post, but the tenon should still be approximately one third the thickness of the rail.

There are many ways to cut these joints.  The mortise can be drilled out and then cleaned up with a chisel, bored with a hollow chisel mortiser, or routed with a plunge router.  Most often I prefer the plunge router for speed and accuracy.  Begin by carefully laying out the position of the mortise in pencil.  After cutting the mortise in the stile or post, position the rail in relation to it and transfer the layout marks and measurements for the tenon onto the stock.  The tenon can be sawn by hand, bandsaw, or tablesaw.  Usually I cut them on the tablesaw.  I prefer to begin by scoring the shoulders at the tablesaw.  Using my crosscut sled with a stopblock clamped to determine the length of the tenon, I set the height of the blade and cut the two sides of the rail.  Next, I use a tenoning jig and stand the rail up and cut away the cheeks.  If needed, I use a sharp chisel to clean up the shoulders.  The strength of the mortise and tenon joint is derived from the relationship of the shoulder and cheek.  Gluing up this joint is pretty straight forward.  You will want to apply glue to the walls of the mortise and brush some on the tenon.  It is a good idea to place some blocks of wood between the clamps and the frame so the clamps do not dent the finished piece.