Two-dimensional Frames (Doors, mirrors, wall panels) Carcasses (Bookshelves, etc.)

Three-dimensional Frames (Tables, chairs, beds)

A barefaced tenon like this is not recommended. Try reducing the tenon thickness to get a shoulder on the bare side.

When you place a mortise near the end of a stile leave an extra inch of so of horn to cut off later.

When joining two pieces of the same thickness, balancing the joint for strength amounts to making the thickness of the two cheeks added together equal the thickness of the tenon.

­­Two-dimensional Frames

Adjust the position of the tenon at the end of the rail to logically suit the location of the rail in the panel.

­­Three-dimensional Frames

With three dimensional frames, design considerations become almost infinitely variable. Regardless of how the joint is employed to create a pleasing appearance, the basic rules of mortise and tenon joinery must still be observed.

Carcases

In this bookcase example, the author recommends a shallow dado stretching between a pair of through mortises.

Over the years the mortise and tenon has assumed a variety of design forms to suit three primary furniture applications: two-dimensional frames, three-dimensional frames and carcases (see below). Regardless, the variations of these joints must follow the design characteristics of the more common forms in order to work properly.

Anatomy of Parts

A key step to understanding mortise and tenon joinery is knowing the parts, many of which are derived from human anatomy.

The specific name of the mortise/tenon piece depends upon its function and orientation. The mortise piece is usually the upright member, such as a stile, leg, or post. The tenon piece is usually horizontal, such as a rail, an apron, a stretcher, or a shelf.

Dimensioning the Parts

On a stopped joint leave a gap between the end of the tenon and the bottom of the mortise. The two parts don’t need to meet since the end grain of the tenon isn’t a gluing surface, and it’s easier to make if you leave a gap of about 1/16".

B = Tenon width = Mortise length
Make the tenon width to exact fit. If it’s too narrow, the tenon may float and position the rail in the wrong place. For the same reason, don’t crush the edges of the tenon during assembly thinking you are making the joint tight. More likely you are causing misalignment. The end walls of the mortise are end grain and therefore not suitable gluing surfaces.

C = Tenon thickness = Mortise width
Make the tenon width to exact fit. If it’s too thin, it will likely float and position the rail in the wrong place. You should be able to “push fit” the parts together. A “clamp fit” with squeaky resistance is too tight. Applying glue will swell an easy-fit joint much tighter. A too-tight joint might split when glue is added.

Geometry of the Parts

Accurate joinery, whether cut by hand or machine, is defined by accurate geometry. Some authors recommend under-cutting the shoulder to ensure a tight-fitting joint. My view is, don’t. The undercut shoulder presents an edge to the stile instead of a flat face. Clamp pressure easily pushes the edge into the stile, distorts the wood, and changes the between-the-shoulders distance. Most important, you also lose the mechanical efficiency of a dead stop.

Mortise and tenon joinery is subjected to two types of stress. One type arises from use, as when we lean back in a chair or push a table to a new location. The other type arises from wood movement. Wood shrinks and expands across the grain in response to changing humidity but not along its length. To cope with both types of stress, we design the joint to be as strong as possible. Since the configuration and section of the parts being joined are usually unique, we must design for the job at hand.

Shoulders

All tenons require shoulders. There are two types. Structural shoulders resist bending stress. Cosmetic shoulders hide the joint if the rail shrinks, and they also hide the gap presented by a damaged mortise end or a mortise made slightly long. Cosmetic shoulders need be only 1/8" to be effective.

A tenon without a shoulder on one side is called a barefaced mortise and tenon. I don’t recommend this configuration. Instead, reduce the thickness of the tenon if you can to get a shoulder on each side.

Proportioning the Joint Parts

If the tenon is too long, the joint is under greater stress due to wood movement. Too short, and it may fail due to too little glue surface. My compromise is to mortise about halfway into the stile. In narrow stock, go a bit more than halfway.

If a mortise extends too near to the end of the stile of a corner joint, the stile may split. Therefore, make the tenon shoulder nearest the end of the stile one-quarter the total width of the tenon. To keep from splitting the mortise, an extra inch or so (called the horn) is left on the stile. Once the glue is cured, saw this horn off.

When the mortise is not at the end of the stile, make the tenon the full width of the rail, less 1/8" at each edge for the cosmetic shoulder.

When joining two pieces of wood of the same thickness, divide the thickness roughly in half to create a balanced joint (see illustrations at right). For example, if the wood is 3/4" thick, make the tenon 3/8" thick and each mortise cheek 3/16" thick. Since each piece has an equal amount of tissue, each is equally able to resist stress. The joint is balanced.

Two-dimensional: Frame and Panel

Two-dimensional frames with a solid panel trapped in a groove make a classic frame and panel. This ingenious construction is not only the foundation of furniture making, it remains the only method we have for making a dimensionally stable frame from dimensionally unstable solid wood. The traditional example is, of course, the frame and panel door. Although with modern material this door is no longer the most efficient design available, tradition and a sense of history continue to drive the popularity of this beautiful cabinet staple.

Three-dimensional Frames

Three-dimensional frames are used to make table bases, chairs, and the skeletons of large cases. Three members generally connect, for example, a table leg and two rails or aprons. In most situations the vertical leg is more or less square in section, while the incoming rails are flat. Factors affecting the design of the joint include the width and thickness of the rails, the exact location of the rails on the faces of the leg, and complications caused by drawers or doors.
The illustrations at right show the rail in three different places on the leg. Once the joint is glued, the “flush rail” must be made dead flush by planing - a manufacturing detail unnecessary with the inset rail. However, the strongest reason to inset the rail is visual: the change in level, along with the associated highlights and shadows, emphasizes the vertical element of the leg.

Note that the thickness of the tenon is increased as it is inset while the gluing area decreases. In each case the tenon is mitered in the middle of the joint. Provided the tenons don’t touch, you can make the gap as small as you like. The 1/8" inside shoulder on the flush and inset rail is enough to do the job.

Carcases

Another place where mortise and tenon joints can occur is in projects such as a bookcase. Here, I use through tenons. The through tenons are both structurally integral and beautiful. An additional joinery technique that I regularly employ is a conjoined housing (or a dado here in America). The 1/8" deep dado shown between the mortises increases the weight-bearing capabilities of the shelf and flattens any cupping in the shelf and upright pieces. It also presents a clean and subtle joint line.