Mortise and Tenon

Part of Woodworking

The mortise-and-tenon joint has been used for at least 7,000 years and remains the strongest way to join two pieces of wood at an angle. Every chair, table, door frame, and timber building in history relied on some version of this joint. Learn it well — it is the backbone of all serious woodworking.

Anatomy of the Joint

The joint has two halves:

• Mortise: A rectangular hole (pocket) cut into one piece of wood — the receiving member
• Tenon: A rectangular tongue shaped on the end of the mating piece — it slides into the mortise

The mortise is always in the piece that stays still (a leg, post, or rail). The tenon is on the piece that connects to it.

The One-Third Rule

The fundamental sizing rule for mortise-and-tenon joints:

Dimension	Rule
Tenon thickness	1/3 of the stock thickness
Mortise width	Matches tenon thickness exactly
Tenon width	2/3 to 3/4 of the stock width
Tenon length	At least 4x the tenon thickness (deeper = stronger)

Example: For a 30 mm thick rail joining a 40 mm thick leg:

• Tenon thickness: 10 mm (1/3 of 30 mm)
• Tenon length: 40 mm minimum (4 x 10 mm), or as deep as the leg allows
• Tenon width: 20-22 mm

Warning

Never make the tenon thicker than 1/3 of the stock. A thick tenon leaves thin mortise walls that split under load. The 1/3 rule balances tenon strength against mortise wall integrity.

Marking Out

Precise layout is the difference between a joint that fits and one that wobbles.

Tools Needed

• Marking gauge (or a scratch gauge made from scrap)
• Try square
• Marking knife (preferred) or sharp pencil
• Ruler or dividers

Marking the Mortise

1. Set the marking gauge to the mortise width (1/3 of stock thickness)
2. Center the gauge on the mortise piece — adjust until the pins are equidistant from both faces
3. Scribe two parallel lines along the length where the mortise will go
4. Mark the mortise length with a square — match the tenon width
5. Scribe end lines with the knife and square

Marking the Tenon

1. Using the same gauge setting (do not reset it), scribe the tenon thickness on the end grain and both faces of the tenon piece
2. Mark the tenon length (shoulder line) with a square all the way around the piece
3. Mark the tenon width — measure in from both edges equally, leaving the same waste (called “cheeks”) on top and bottom

Tip

Always use the same gauge setting for both pieces. If you reset the gauge between mortise and tenon, any tiny error doubles. Set it once, cut both.

Cutting the Mortise

Cut the mortise first. It is harder to adjust, so you fit the tenon to the mortise, not the other way around.

Chisel-Only Method

1. Secure the workpiece in a vise or clamped to the bench
2. Start in the center of the mortise, not at the ends
3. Drive the chisel (bevel facing the center) straight down, about 3 mm deep
4. Move 3 mm toward one end, drive again — the waste chips pop up
5. Work toward the end in 3 mm steps, stopping 2 mm short of the layout line
6. Reverse direction, work toward the other end
7. Deepen by repeating the sequence, going 3 mm deeper each pass
8. Clean the ends last — pare carefully to the layout lines with the chisel held vertically

Drill-and-Chop Method (Faster)

1. Select a drill bit matching the mortise width
2. Drill a series of overlapping holes down the center of the mortise to full depth
3. Chop the remaining waste with a chisel — the holes have removed most material
4. Pare the walls flat and the ends square

Tip

The drill-and-chop method saves enormous time on deep mortises. A brace and bit removes material 5x faster than chiseling from scratch.

Cleaning Up

• Use a chisel the exact width of the mortise to pare the walls flat
• Check squareness by inserting a scrap piece and sighting for gaps
• The mortise walls must be flat and parallel — any twist prevents the tenon from seating

Cutting the Tenon

The tenon is all saw work, refined with a chisel.

Step-by-Step

1. Clamp the workpiece upright in a vise, end grain facing up
2. Saw the cheeks: Cut on the waste side of the gauge lines, angling the saw to follow the line down both faces. Then level the workpiece and saw straight down to the shoulder line. You are removing wood from both sides to form the tenon thickness.
3. Saw the shoulders: Lay the piece flat. Cut on the waste side of the shoulder line across the full width. The cheek waste falls away.
4. Trim the tenon width: If the tenon is narrower than the stock, saw the top and bottom edges the same way — saw to depth, then crosscut the shoulder.
5. Pare to fit: Test the tenon in the mortise. It should slide in with hand pressure — not forced, not loose. Pare with a chisel to remove any high spots.

Warning

Always saw on the waste side of your line. You can pare wood away but you cannot add it back. A tenon that is 0.5 mm too thin will never make a tight joint.

Test Fitting

1. Dry-fit the joint without glue
2. Check for gaps at the shoulders — the tenon shoulders should seat firmly against the mortise piece face
3. Check alignment — sight along both pieces to ensure they are in the same plane
4. Mark any high spots where the tenon binds before the shoulder seats
5. Pare and re-test until the fit is snug with no visible gaps

The joint should hold together under its own friction. If it falls apart under gravity, the tenon is too thin.

Mortise-and-Tenon Variations

Through Mortise

The mortise goes completely through the receiving piece. The tenon end is visible on the far side.

• Advantages: Stronger (more glue surface), tenon end can be wedged, visually confirms joint is fully seated
• Disadvantages: End grain visible, requires clean work on both sides

Blind (Stopped) Mortise

The mortise stops partway through. The tenon is hidden.

• Advantages: Clean appearance, end grain hidden
• Disadvantages: Less glue surface, harder to verify full seating, trapped air can prevent seating (solution: cut a small groove in the tenon for air escape)

Haunched Tenon

A step is cut in the tenon, leaving a short stub (the haunch) that fills a shallow groove in the mortise piece.

• Purpose: Prevents the mortise piece from twisting. Used at the top of a frame where the mortise would otherwise break through the end grain.
• Haunch depth: Usually 1/3 of the tenon length
• Common use: Door and window frames

Wedged Tenon

For through mortises, saw two kerfs in the tenon (parallel to the grain), then drive thin wedges into the kerfs after assembly. The tenon end flares inside the mortise, creating a permanent mechanical lock.

1. Cut kerfs about 2/3 of the tenon length, spaced evenly
2. Start assembly — slide the tenon through the mortise
3. Apply glue to the wedges
4. Drive wedges firmly — the tenon spreads and locks
5. Let dry, then flush-cut the protruding tenon and wedge ends

Tip

For an extra-strong wedged joint, slightly flare the far end of the mortise (1-2 mm wider). This gives the wedged tenon room to expand and creates a dovetail-like lock.

Drawbored Joints

Drawboring is the most ingenious technique in traditional joinery. An offset peg hole pulls the joint tight without clamps or glue.

How It Works

1. Assemble the joint dry and drill a hole through the mortise walls and tenon
2. Disassemble the joint
3. Re-drill the hole in the tenon only, shifting it 1.5-2 mm closer to the tenon shoulder
4. Reassemble and drive a wooden peg through all three holes

The offset forces the tenon forward as the peg pushes through, pulling the shoulder tight against the mortise face. The joint is permanently locked by the bent peg.

Rules for Drawboring

Parameter	Guideline
Peg diameter	1/3 to 1/2 of tenon width
Offset distance	1.5-2 mm (more risks splitting)
Offset direction	Toward the shoulder (pulls joint tight)
Peg material	Straight-grained hardwood, slightly tapered tip
Peg moisture	Drier than the frame (swells to lock)

Common Mistakes and Fixes

Mistake	Cause	Fix
Tenon too loose	Over-cut cheeks	Glue a thin shim to one cheek, re-pare to fit
Shoulder gaps	Tenon bottoms out before shoulder seats	Shorten tenon by 1-2 mm, or deepen mortise
Mortise walls not parallel	Chisel twisted during chopping	Pare walls flat with a wide chisel, checking with a straight edge
Joint twists out of plane	Mortise not square to face	Re-cut mortise with careful square reference
Tenon splits when driving	Tenon too tight	Pare cheeks — never force a tight tenon
Mortise splits at end	Chopping too close to end grain	Leave at least 30 mm of material beyond the mortise end

Warning

The most common beginner mistake is cutting the mortise too close to the end of a piece. End grain is weak. Always leave at least 30 mm (more in softwood) between the mortise end and the piece end, or the mortise wall will split when you drive the tenon home.

Mortise and Tenon — At a Glance

Size the tenon at 1/3 of stock thickness. Cut the mortise first using chisel-only or drill-and-chop. Cut the tenon with saw kerfs and pare to a snug hand-pressure fit. Choose through, blind, haunched, or wedged variations based on your application. For permanent joints without glue, use drawboring — offset peg holes pull the shoulder tight and lock mechanically. Master this single joint and you can build furniture, doors, frames, and timber structures that last centuries.