What are Mortise-and Tenon Joints Used for?

Mortise-and-tenon joints provide a strong, robust connection that can last for centuries. This durable and popular joint has been used to build everything from neolithic stonework to the Notre Dame Cathedral in Paris to IKEA furniture.

What Is a Mortise-and-Tenon Joint?

The easiest way to think of this joint is as an “insert tab A into slot B” connection. The mortise is a slot carved into the receiving material, while the tenon is a tab, slightly reduced from the inserted material’s original size. That reduction means there’s a shoulder around the tenon, giving added stability in all directions.

The tenon should fit snugly into the mortise, creating a joint that can be held by friction, as well as with adhesives or fasteners.

Some mortise slots are open, allowing the tenon to pass through the material, while others hide the end of the tenon inside the mortise space. Mortise-and-tenon joints go into many types of projects, from rustic carpentry to fine furniture making.

Types of Mortise-and-Tenon Joints

Terms to describe the various kinds of mortise-and-tenon joints vary by region and even from woodworker to woodworker. Here’s an overview of the four most common variations a beginner woodworker may encounter.

• Blind joint: Also known as a “stub” or “hidden” joint. In this one, the mortise does not pass all the way through the mortised material. That creates a pocket for the tenon to rest in, concealing the end grain of the tenon.
• Through joint: Conversely, a “through” or “open” joint features a mortise passing all the way through the material, like the eye of a needle. This allows the tenon to end flush with the outer face of the mortised material or extend beyond it.
  
• Wedged tenon: Some through mortise-and-tenon joints add strength by widening the end of the tenon that extends through the mortise. These wedges give the tenon a dovetail shape, which the outer face of the mortise is cut to accommodate. The wedges can be applied in kerfs cut into the tenon, or glued in place on both edges of the tenon.
• Tusked tenon: This was a popular type of through tenon in antiquity, and is still used in some furniture designs today. The tenon extends past the end of the mortise material, and a hole added to the side of the tenon accepts a wedge or pin. This functions like a cotter pin, preventing the tenon from slipping out.

Mortise-and-Tenon Joint Advantages and Disadvantages

Like all joinery, mortise-and-tenon joints are not perfect in all situations.

Advantages

• Extremely strong: Whether on their own or reinforced with glue or pins, they’re difficult to separate.
• Classic design: Blind mortise-and tenon joints hide the end grain of the tenon, allowing you to highlight the natural beauty of the material. Through mortise-and-tenons joints have their own charm, with a distinctive look that can still be finished to a level that makes clear the quality of craftsmanship.
• Versatile: As mentioned, mortise-and-tenon joints can be used in many types of projects. Once you master them, you’ll be amazed at how often you use them.

Disadvantages

• Intimidating to beginners: Many woodworking newbies find mortise-and-tenon joints intimidating. They’re actually not difficult to make once you’ve attempted a few.
• Not suitable for thin material: They’re not as good for smaller material sizes. There’s no exact point to walk away from them, but at a certain point the whittling down of toothpick-sized sticks into tenons is a waste of time.

Mortise-and-Tenon Joint Size

Start with determining the mortise slot’s dimensions.

• Mortise hole width: Should be no more than one-third the width of the mortise material. If you’re mortising a 1×4 (1x material is 3/4-in. thick), the mortise hole should be 1/4-in. thick, usually situated in the middle of the mortise piece with 1/4-in. of material on either side. In some situations the mortise needs to be offset, but that’s a more advanced joinery topic.
• Mortise hole lengths: Should be between one and four times the hole’s width, ranging from a square to a rectangle, or a circle to an oval. Anything more than that should be separated into multiple tenons.
• Mortise hole depth: A blind mortise should be one-half to two-thirds of the mortised material’s depth. There is no depth for a through mortise, because by definition it goes all the way through the material.
• Tenon sizing: Determined from the above measurements. Check the tenon measurements against the size of the stock you’re using. If you’re joining same-sized material, there’s no problem. But if the tenon material is smaller than the mortise material, you’ll have to adjust accordingly. A 4×4 being mortised would have a slot 1.16 inches wide, something that clearly won’t work if you’re using a 1×4 board for the tenon material.

Many sources recommend making the mortise first, then the tenon. You can make them in whatever order you like, but for beginners, it’s often best to stick with the mortise hole first. This is because mistakes happen, especially as you’re learning, and repairs to a tenon are simpler and much less noticeable than those to the mortise hole.

Mortise-and-Tenon Joint Tools

There are lots of ways to make a mortise and tenon joint. Here are some of the most common favored by DIYers, and the tools required make them.

• Chisels can do the job from start to finish, or just to clean up and fine-tune.
• Drills are the choice for many beginners. Because the bottom of the mortise hole will be flat, it’s often easier to use a Forstner bit. Mortising jigs are available for handheld drills and drill presses.
• Plunge routers work great, as long as you’re careful to not take away too much material in one pass.
• Hand saws, like this specialty tenon saw, are good if you’re on a budget or making cuts on thin material.
• Table saws, combined with a jig for cutting vertical material, can make short work of tenon-making.
• Marking gauges with double pins can make laying out the outline for your mortise-and-tenon cuts easier.