Newsletter Article: Type of Mortise and Tenon Joints

Types of Mortise and Tenon Joints

By Luther Shealy

 

When building a strong structure or a piece of furniture, the joints you use matter. One of the most common and strongest joints is a mortise and tenon joint, an extremely old woodworking technique that has stood the test of time and is still used today.  In its most basic form, a mortise and tenon joint is simple, extremely strong, and the technique can be scaled up or down in size with great success.  

 

There are many variations of the mortise and tenon joint, but the basic joint comprises two components:

  • The mortise hole, and
  • The tenon tongue.

 

The tenon, formed on the end of a member generally referred to as a rail, fits into a square or rectangular hole cut into the other, corresponding member. The tenon is cut to fit the mortise hole exactly. It usually has shoulders that seat when the joint fully enters the mortise hole. The joint may be glued, pinned, or wedged to lock it in place.

 

The mortise and tenon is an ancient joint dating back 7,000 years. The first examples, tusked mortise and tenon joints, were found near the German city of Leipzig - the world's oldest intact wooden architecture. 

 

 

The mortise and tenon joint was used to join wooden planks of the "Khufu ship", a sailing vessel found sealed in a pit in the Giza pyramid complex of the Fourth Dynasty around 2500 BC.  It has also been found in ancient furniture from archaeological sites in the Middle East, Europe and Asia.  The ancient Romans made heavy use of the mortise and tenon joint in wood-frame construction and stonework and they built more temples, houses, forts, walls, aqueducts, arches, roads, and palaces than anyone in the ancient world.  In traditional Chinese architecture, wood components, such as beams, brackets, roof frames and struts, were made to interlock with mortise and tenon joints, without using fasteners or glues, enabling the wood to expand and contract according to humidity.

 

Building a solid mortise and tenon joint requires a high degree of precision.  To achieve a strong joint, the tenon must fit snuggly into the mortise (requires hand pressure to insert the tenon into the mortise) along the width of the timber.  It is this snug fit that creates friction between the mating pieces of wood that resists the joint from pulling apart. 

 

 

If applying glue to the joint, this snug fit is needed to allow for proper glue adhesion to bind the two pieces of wood together.  If the tenon width is too narrow there will be no friction to resist tension nor will there be enough surface to surface contact for a good glue joint.  If the tenon’s width is too thick (must beat the tenon into the mortise with a mallet) you risk blowing the joint apart when the wood naturally expands and you may starve the joint of glue, if using glue. 

 

When creating a mortise and tenon joint you should cut the mortise first and then cut the tenon to fit, rather than the other way around. If adjustments need to be made, it’s much easier to alter the size of tenon than the mortise.

 

The mortise can be cut all the way through the timber, making it an “open” or “through” mortise or it can be cut less than all the way through the timber, making it a “hidden” or “blind” mortise.

 

 

 

The tenon can be made to pass all the way through the mortise, making it a “through” tenon, or the tenon can be made to be enclosed by the mortise, making it a “stub” or “blind” tenon.

 

 

 

Over time craftsman have developed general rules for properly sizing their mortise and tenon joints, they are: 

 

Tenon length: A tenon's length should be at least five times its thickness. So, a 1/4″-thick tenon should be 1-1/4″ long. Of course, if you look at antique furniture, you see this “rule” violated a lot, or maybe the furniture was made before they made the rule!  The point is, these rules are just a guideline.

 

Tenon thickness:  A tenon’s thickness should be one-third the thickness of the stock being mortised. So, if you are joining two pieces of 3/4″material, the tenon should be ¼” thick (1/3 of ¾). If you are joining a 7/8″-thick apron to a 1-1/2″-thick table leg, the tenon should be 1/2″ thick (1/3 of 1-1/2).

 

Tenon Width: This rule is two-fold. First, make the tenon one-half the width of the rail you’re cutting it on (a 2″- wide rail would get a 1″- wide tenon). Second: If that tenon’s width would be greater than six times its thickness, then you should split it into two (or more tenons). Example: You want to cut a 1/4″-thick tenon on a 6″-wide rail.  The rule says that your tenon should be 3″ wide. But a 3″-wide tenon is greater than 1-1/2″, which is six times the tenon thickness. So, you must break that tenon into two 1-1/2″-wide tenons.

 

There are literally hundreds of variations of the mortise and tenon joint, too many to describe here.  However, below is a list of the common types of mortise and tenon joints found in most woodworking.

 

Through Mortise-And-Tenon Joints

 

 

  This is an extremely common joint that is formed by cutting the mortise completely through the stile and sizing the tenon to match, flush with the far side of the stile.

 

The end of the exposed tenon can be further secured by adding a very narrow slot, either diagonal or straight across the width of the tenon, then inserting a solid wedge into the slot once the joint has been assembled. This can greatly strengthen the joint's holding power, but care should be taken to not split the joint apart by using an overly thick wedge. See wedged mortise-and-tenon joints below

 

Blind Mortise-And-Tenon Joints

 

 

 

The blind mortise-and-tenon joint gives the outward appearance of a butt joint, however has all the strength and advantages of a mortise-and-tenon joint. The mortise does not extend completely through the stile, and therefore the tenon is not visible once the joint has been assembled. This is the most commonly-seen version of the mortise-and-tenon joint in use today.

 

Haunched Tenon Joints

 

 

A haunch is a short tongue that protrudes from the rail's shoulder, between the rail's edge and the tongue's edge. When a mortise and tenon joint is constructed with a stile that has a groove through which a tenon is cut, such as in frame-and-panel construction, the normal technique for forming a mortise and tenon would leave a void at the end of the slot, and this void would be visible on such assemblies as panel doors. In order to compensate for the slot, a haunched tenon is constructed so the haunch fills the groove at the tenon's edge. Depending on the final use, the rail can have a haunch on only one side, or both sides, as required.

 

Open Slot Mortise-And-Tenon (Bridle Joint)

 

 

A bridle joint’s distinguishing feature is that the top of the mortise is open.  There are a number of variations on the bridle joint including the corner bridle, the miter bridle, and the T-bridle which joins the end of one member to the middle of another.

 

Wedged Mortise-And-Tenon Joints

 

Mortise-and-Tenon joints can be further strengthened by the addition of a wedge. A thin kerf slot is cut into the end of the tenon, then after the tenon is inserted into the mortise, a wedge is inserted into the slot to secure the joint. Wedged joints such as these may not even require any glue, especially if the mortise is tapered to be wider at the wedge end, so that the joint cannot be pulled apart by brute force.

 

Tusk Tenon Mortise and Tenon Joint

 

 

A tusk tenon creates one of the strongest mortise and tenon joints and it has the benefit of being able to be disassembled.  The tenon is cut to protrude beyond the end of the mortise.  As vertical slot is cut into the end of the tenon through which correspondingly shaped wedged key is inserted.  Tapping the key pulls the tenon shoulders tight against the mortise, locking the two together.

 

Pegged Mortise-And-Tenon Joints

 

 

 

An alternative to wedges, a pinned or pegged mortise-and-tenon joint is extremely strong. After glue-up, drill one or more evenly spaced holes from face-to-face through the stile, close enough to the rail to pass through the tenon, about half-way down its length. Then glue and insert dowels or pegs. These pegs can be made of the same wood to hide the reinforcement, or they can be of a decorative contrasting wood. After the glue has set, the pegs can be trimmed flush with the face.

 

Loose Tenon Joints

 

 

 

Loose tenon joints are constructed by mortising both the side of the stile and the end of the rail, and then inserting an appropriately sized tenon during glue-up. Even though the tenon is not integral to either piece, it still creates plenty of long grain to long grain glue surface to create a very strong joint.

 

Double Mortise-And-Tenon Joints

 

 

For rails that are more than ten times their thickness, multiple tenons should be used. This means that a standard 3/4-inch-thick board move than 7 1/2-inches thick should be treated this way. The proportion between the tenon and the spaces between is the key, and not the actual number of tenons. It is recommended that the space be divided into thirds, two-thirds being tenon and one-third being space, equally distributed along the end of the rail. If the rail is of a material that is prone to cupping or warping, you can leave a short haunch between the tenons and notch the mortise to match.

 

So how do you choose what mortise and tenon joint to use?  For beginners this can be a very difficult decision.  Until you get some experience in the proper joint for the proper application, here are a few rules that may help:

 

  • Keep it simple - Sometimes the simplest joint can do the job . There's no need to complicate things, and sometimes overdoing It can be detrimental. For instance, you don't have to pin every mortise-and-tenon Joint; glue usually is enough to ensure a strong bond.

 

  • Stick with what has worked historically - in woodworking, there's often a good reason why things have been done a certain way for hundreds of years. Woodworkers sometimes struggle to reinvent a joint by exaggerating a part of it. Study well-regarded pieces to follow as examples. The surest way to build successful furniture is to copy what has worked.

 

  • Make sure it fits properly - If you've bothered to make it, then make sure it fits well. Sloppy joints will come apart prematurely when the glue fails. Take your time, test-fit your pieces, and replace anything that's loose or cracked.

 

  • Make sure the work is clean - Every piece you build should represent the best you are capable of building at the time. You shouldn't look at your work and think you could have done better. Sloppy work and shortcuts will haunt and trouble you forever.

 

Four Criteria for Choosing a Joint:

 

  1. The joint should contribute to the strength and Integrity of the piece.

 

  1. The joint should allow the wood to move as ambient conditions change.

 

  1. The joint must allow for additional operations, such as grooving, rabbeting, moldings, or screws.

 

  1. The joint should contribute to and not interfere with the appearance of the piece.

 

Good Luck with your mortise and tenon joints

Luther