What is a Mortise and Tenon Joint?

Mortises and tenons make up one of the most important joints in woodworking. Mortise and tenon joinery is used in all types of furniture making. From frame-and-panel doors to table legs and aprons, mortise-and-tenon joints have been hard at work for hundreds of years. Mortise and tenon joinery is primarily used in solid wood woodworking to join end grain to edge grain. A mortise and tenon joint is, at its most basic, a peg fit into a hole. We show you the parts of a mortise and tenon joint and how it should fit together.

History of Mortise and Tenon Joinery

The mortise and tenon joint has been a cornerstone of woodworking and furniture making for centuries, with evidence of its use dating back to ancient Egyptian and Chinese civilizations. This timeless joint's popularity stems from its strength, versatility, and aesthetic appeal, making it a go-to choice for woodworkers worldwide.

One of the critical turning points in the history of woodworking came when mortise-and-tenon joints were developed. Before that, tables were slabs of wood balanced on pedestals, and wooden doors as well as chest sides and ends were wide slabs of wood often reinforced with metal straps. The ability to strongly join the end of a board to the edge of another board changed the game completely.

Wood movement — the eternal gremlin complicating wooden constructions — could now be managed in frame-and-panel doors. The wide panel floated in grooves in the frame, allowing for seasonal wood movement. The frame, securely connected by mortise-and-tenon joints, held the whole construction together.

The same concept allowed much stronger and more elegant panels for chests and other boxes. Leg-to-apron joints were also a big step up from large and bulky pedestals. The possibilities that this joinery method opened up were legion.

Parts of a Mortise and Tenon Joint

Let's take a closer look at a mortise and tenon joint. The tenon is this rectangular portion on the end of the workpiece. It typically has four shoulders that wrap all the way around it, they're these horizontal portions. The vertical portion of the tenon is called a "cheek". Now, tenons fit into mortises on the mating workpiece. A mortise is nothing more than a four-sided hole that's sized to fit the tenon. Now, on a typical mortise and tenon joint, the thickness of the tenon is about one third the thickness of its workpiece.

While there are many different tenon styles — split tenons, though tenons, slip tenons in bridal joints — the basic concept is a rectangular segment machined into the end of a board, often called a rail. Tenons are specifically made to fit the mortise, a rectangular hole in the edge grain of another board, termed a stile. Best practice is to make the mortises first and then form the tenons to fit.

Tenons can be thicker than one third with narrower shoulders so long as the mortise workpiece is thicker, so that its strength won't be compromised.

The tenon shoulders prevent the tenon from ceding completely in the bottom of the mortise. That leaves some clear space at the bottom of the mortise for glue to migrate. Shoulders also add some lateral stability to the joint and they completely hide the mortise when the joint is assembled. That's why when you're making mortise and tenon joints, you want the shoulders to line up all the way around the tenon, and you want the cheeks of the tenon to fit snugly against the walls of the mortise. A well-made mortise and tenon joint should fit together like a piston and a cylinder.

How are Mortise and Tenon Joints Made?

Hand Cutting Mortise and Tenon Joints

Hand-chopping a mortise is a tried-and-true method, as effective today as it was a hundred years ago. Mortising chisels are heavy-duty tools that are driven with a mallet. Then the wood is pried out.

The traditional method hand cutting a mortise, using a mortising chisel to form the rectangular opening was a fairly simple process to master. Raising the tenon using a hand saw and chisel required more skill, but it was within the reach of nearly all woodworkers, requiring only three basic tools for the entire operation. And while those methods continue to work really well today, they are time-consuming and take a bit of trial-and-error practice.

Machine Cutting Mortise and Tenon Joints

The Benchtop Mortising Machine is a fast and consistent way to form mortises. It uses a specially-made four-sided mortar chisel bit. Inside the chisel, an auger bit is used to bore into the wood and remove the waste. These machines work remarkably well, although care must be taken to get good results.

Cutting tenons with a table saw is a fast and consistent method to efficiently form tenons regardless of stock thickness.