What is a Mortise?

Mortises at a Glance

A mortise is the rectangular pocket cut into a piece of wood to receive a projecting tab (the tenon) from another piece. Slide the tenon into the mortise, apply glue to the long-grain walls, and you have a joint that held Stonehenge together and still locks table legs to aprons in furniture built today.

Click to expand

In this guide:

• What a mortise is and how it works
• Why mortises outlast every other joint
• The three types: blind, through, and open
• Sizing a mortise: the rule of thirds
• Where mortises show up in real furniture

Part 1: What a Mortise Actually Is

A mortise is a rectangular slot cut into a piece of wood. Picture a rectangular hole (about the width of a chisel blade, an inch or so deep) punched into the face of a board or the side of a leg. That's the mortise. The tenon is the matching tab cut on the end or face of a second piece. One slides into the other. Together they form the mortise-and-tenon joint.

Click to expand

The easiest analogy: a mortise lock. That brass mechanism set into the edge of your door sits in a mortise, a rectangular pocket routed into the door stile. The woodworking joint works the same way, just made of wood.

Physically, a mortise has four walls (two long sides, called cheeks, and two short ends) and, for the most common type, a flat closed bottom. Width runs from 1/4 inch in furniture made from 3/4-inch stock up to an inch or more in heavy timber work. Depth typically ranges from 3/4 inch to 2 inches depending on the joint's job.

Why the shape matters

The rectangular cross-section does something a round hole can't: it creates long-grain-to-long-grain contact between the tenon's cheeks and the mortise's walls. Long grain glue bonds (where wood fiber runs parallel to the glue line) cure harder than the wood itself. End grain bonds are weak because the open cell structure drinks up the glue instead of bonding with it.

A mortise and tenon joint doesn't rely entirely on glue. The tenon is trapped mechanically. It can't pull straight out, rack sideways, or twist without shearing the wood on both sides. The glue and the geometry reinforce each other, which is what makes this joint survive furniture that everything else about the piece doesn't.

Part 2: Why Mortises Outlast Every Other Joint

There are dozens of ways to join two pieces of wood. Pocket screws, biscuits, dowels, splines, dovetails. Each has its place. The mortise and tenon dominated structural joinery for 7,000 years because nothing held as well.

7,000 years of field testing

The oldest mortise-and-tenon joints on record come from water wells near Leipzig, Germany. Wikipedia's mortise and tenon article cites tree-ring analysis and carbon dating placing those wells between 5,600 and 4,900 BCE. Neolithic timber construction, with joints you'd recognize on a workbench today.

The Khufu ship, a 43-meter cedar vessel sealed into a pit at Giza around 2,500 BCE, used mortise-and-tenon joints to connect its planks. The sarsen stones at Stonehenge were shaped with mortise-and-tenon joints before being raised (2,600–2,400 BCE). Stone mortised onto stone. The joint worked the same way in stone as in wood.

The numbers

Woodgears.ca's joint strength test pitted a mortise-and-tenon joint directly against a dowel joint of equivalent size: same spruce frame, same yellow glue, same drying time. The mortise-and-tenon failed at an average of 172 pounds of force; the dowels gave out at 135 pounds. That's roughly 25 percent stronger, and the test didn't use particularly thick tenons.

Research published in BioResources journal confirmed the same pattern: mortise-and-tenon joints consistently outperform comparable joints in both strength and stiffness tests, with double mortise-and-tenon configurations performing better still.

Click to expand

Three things make it strong:

1. Glue surface area. The tenon's long-grain faces bond to the mortise walls. More face contact means more holding power.
2. Mechanical interlock. The tenon can't pull out without shearing the wood around it. Glue failure alone doesn't kill the joint.
3. Racking resistance. A table with mortise-and-tenon apron joints resists the lateral force when you lean on it. A table with butt joints and screws doesn't.

Part 3: The Three Types of Mortises

All mortises cut a rectangular slot into wood. Where they differ is whether that slot has a bottom, passes all the way through, or stays open on one end.

Click to expand

Blind mortise

The blind mortise (also called a stub mortise) is the most common type in furniture making. The slot has a closed bottom. It goes partway into the wood, stops, and you never see the end of the tenon from the outside. The finished joint looks clean on every face.

The standard depth: 2/3 the width of the board being mortised. For a 1-1/2-inch wide apron, that's a 1-inch deep mortise. It's a minimum. Deeper is fine as long as you leave enough material at the bottom.

Blind mortises show up in table aprons, chair rails, cabinet face frames, and door stiles, wherever you want the joint invisible from the outside.

Through mortise

A through mortise passes entirely through the piece. The tenon emerges on the far face and can be left flush, trimmed proud and planed smooth, or extended further and wedged for a mechanical lock that works even if the glue ever lets go.

Through mortises are stronger than blind mortises because the tenon can be as long as the full thickness of the mortised piece. No depth restriction. They appear in workbenches, trestle tables, Arts & Crafts furniture (where the exposed tenon end is a design feature), and timber framing.

The wedged through tenon is a variation worth knowing. You saw a kerf in the tenon end, slide in a wooden wedge after assembly, and drive it home. The joint tightens under load. Timber framers used this for centuries before metal hardware existed.

Open mortise

An open mortise has only three walls. The slot is open on one end, which creates what's called a bridle joint or slip joint. It's less common, used mainly in chair top rails, gate construction, and applications where the joint assembles from above rather than sliding together laterally.

Because one end is open, an open mortise is weaker than a blind or through mortise in tension. It's held in place mainly by the shoulders of the tenon and the geometry of the joint, making it better suited to applications where the joint stays in compression.

Part 4: Sizing a Mortise

Get the proportions right and the joint is strong. Get them wrong and you either weaken the tenon (too wide a mortise) or risk blowing out the mortise walls (too narrow a mortise leaves insufficient material on each side).

The rule of thirds

Mortise width equals one-third of the stock thickness being mortised. Start here.

The math is simple: a 3/4-inch thick board gets a 1/4-inch mortise. A 1-1/2-inch leg gets a 1/2-inch mortise. The remaining 1/3 on each side of the mortise gives the walls enough strength not to split under load.

Click to expand

Source: Woodworkers Journal's mortise and tenon dimension guide

Depth, length, and width limits

Depth (blind mortise): depth = 2/3 of the width of the board being mortised. If the mortised board is 1-1/2 inches wide, the mortise goes 1 inch deep.

Tenon length: The tenon should be at least five times its thickness. A 1/4-inch thick tenon needs at least 1-1/4 inches of length to have meaningful glue surface. Longer is better, up to the limits of the blind mortise depth rule.

Tenon width: Keep it under 4 inches wide. A wide tenon that can't move seasonally will crack the mortise walls. If your rail is wide, use two narrower tenons instead of one big one. Popular Woodworking's tenon sizing guide puts the conservative max at 4 inches, with 6 inches as a realistic outer limit for stable, kiln-dried stock.

Cut the mortise first. Always. A mortise is hard to adjust once cut. Widening or lengthening it precisely takes real effort. A tenon can be shaved with a shoulder plane or chisel in seconds. Cut the mortise to your target dimensions, then trim the tenon until it fits. Katz-Moses Tools' mortise and tenon guide calls this out as the single rule beginners most often break.

Part 5: Where Mortises Show Up in Real Furniture

Tables

The classic application: apron to leg. The apron (the horizontal rail that runs between the legs under the tabletop) has a tenon on each end. Each leg has a mortise cut into its inner face to receive that tenon. This is the joint that keeps a table from racking when someone leans on it from the side.

Typical dimensions for furniture-weight stock: 3/4-inch apron, 1/4-inch tenon, fitting into a 1/4-inch mortise cut into a 1-1/2-inch leg, 1 inch deep. Four of these joints, one at each leg, lock the table's frame into a rigid structure.

Click to expand

Chairs

Chairs are the most demanding mortise-and-tenon application. A chair absorbs racking forces from every direction as people lean back, rock, twist, and shift. Chair rails (front, back, and side stretchers) join to the posts with mortise-and-tenon joints at angles that aren't always 90 degrees. The mortise itself is identical to one you'd cut for a table apron. The challenge is the compound angles at which they meet.

Doors and windows

Traditional door construction uses mortise-and-tenon at every stile-to-rail joint. The stile (vertical piece) holds the mortise; the rail (horizontal piece) holds the tenon. A variation called the haunched tenon fills the groove cut for the door panel, preventing a gap at the corner. This is standard construction in exterior doors built to last decades.

Workbenches

A Roubo or Nicholson workbench uses through mortise-and-tenon joints in the stretchers connecting the legs. These are large joints, sometimes an inch or more wide, with through tenons wedged from the outside. The bench takes impact from planing, sawing, and chiseling; it needs joints strong enough to absorb that abuse without loosening over time.

RELATED: Dovetail Joint Another structural joint worth knowing. Dovetails resist pull-apart forces that mortise and tenon joints aren't designed for.

The next step is cutting one. A sharp chisel, a mallet, and a drill to hog out the waste gets you there without specialized equipment. If you have a drill press and a Forstner bit, the Rockler drill press mortising technique works well: drill overlapping holes to clear the bulk of the waste, then square the walls with a chisel.

Sources

Research for this guide drew on woodworking publications, joint strength tests, and historical documentation of mortise-and-tenon construction.

• Wikipedia — Mortise and tenon — history, types, global examples
• Laurel Crown — The History of Mortise and Tenon — historical context
• Woodgears.ca — Mortise and Tenon vs. Dowel Strength Test — comparative strength data
• BioResources — Strength and stiffness analyses of standard and double mortise and tenon joints — peer-reviewed structural data
• Woodworkers Journal — Real-life Mortise and Tenon Dimensions — sizing rules and practical examples
• Popular Woodworking — Tenons Rule! So Here Are the Rules on Tenons — tenon width and length guidelines
• Katz-Moses Tools — Cut Better Mortise and Tenon Joints — layout strategy and beginner tips
• Rockler — Cutting Mortises with a Drill Press — hybrid drill press method
• Woodsmith — Cutting a Perfect Mortise — layout and execution detail
• Family Handyman — What Is a Mortise-and-Tenon Joint? — furniture applications
• Britannica — Mortise and tenon — authoritative overview
• Kreg Tool — What Is a Mortise and Tenon Joint? — beginner-oriented explanation