Table of Contents
A mortise is a rectangular hole that must be square in two directions, parallel in two more, and sized to a tenon you have not cut yet. A mortiser or router will cut one faster; what hand chopping offers is feedback: the chisel tells you when a wall is drifting, when the grain is about to tear, and when the bottom has gone below level.
Most of the work happens before the edge touches wood. Accurate layout on both faces, knife walls that register the chisel back, and relief holes that remove most of the waste turn a nervous job into a fifteen-minute one.
The sequence below is for a blind mortise 3/8 in (10 mm) wide, 2-1/2 in (64 mm) long, and 1-3/8 in (35 mm) deep, sized for a tenon 1-1/4 in (32 mm) long. Change the dimensions for a different joint, but keep the order: layout, knife, bore, chop, pare, fit.
Establish the Reference Faces First
Every line on a mortise comes from two surfaces: the face side and the face edge. Mark them with a chalk triangle before gauging. The fence of the mortise gauge rides on the face side, and the stock of the square sits against the face edge. Gauge from the opposite face and the mortise shifts by whatever thickness variation the board carries, which pulls the assembled joint out of square.
Set the spurs to the chisel's actual width, measured across the back near the edge rather than trusting the stamped size. A 3/8 in chisel that measures 0.372 in needs a mortise set to 0.372 in. Set the spurs wide by 1/64 in and the chisel drops in easily, but the walls no longer support the tenon and the joint depends on glue rather than wood. The full procedure for setting the fence and spurs is covered in setting a mortise gauge.
Carry the lines around the end grain and down the far face with a square. A blind mortise needs lines on the entry face, but lines on the far face reveal a chisel that has drifted. Mark the depth directly on the chisel shank with tape so you can see it without breaking rhythm.
Cut Knife Walls, Not Pencil Lines
A knife line severs the surface fibers and gives the back of the chisel a seat. A pencil line is a suggestion; a knife wall is a register. Score each line with a marking knife held against a straightedge, two light passes rather than one heavy one, until the cut is about 1/32 in (0.8 mm) deep. The long walls can be scored with a marking gauge; the ends need a knife and square.
On end grain, score from both directions so the fibers at the corner are cut rather than pushed. The knife wall stops the chisel crushing fibers outside the mortise and gives the flat back a seat so the first blow goes down the line.
Bore Out the Bulk of the Waste
For a 3/8 in mortise, use a 5/16 in auger bit or a 3/8 in Forstner bit that you can keep centered. Bore overlapping holes along the centerline, spaced roughly one bit diameter apart, from the entry face to a depth 1/8 in (3 mm) shy of the final bottom. Leave 1/8 in of wood at each end and about 1/16 in (1.6 mm) on each wall.
The relief holes change the mechanics of the cut. A chisel removing a solid block must break the whole mass loose with nowhere to push it. A chisel working against a bored cavity only has to shear a thin web, and the chip has an open space to fold into. Roughly two-thirds of the waste disappears before the first mallet blow.
If the mortise goes through the workpiece, stop the bit 1/8 in short of the exit face and finish the last of the waste with the chisel from the other side.
Chop in Thin Bites From the Ends
Clamp the work at a comfortable height and stand slightly to the side, so your shoulder, elbow, and wrist line up over the blade.
Start at the far end of the mortise with a mortise chisel. The flat back faces the end line and registers against it; the bevel faces the waste in the middle. Strike with a mallet and take a bite no deeper than 1/8 in. Lever the handle toward the center of the mortise to break the chip free, never sideways against a wall. Work the far end, then the near end, then the middle, and repeat at the next depth.
The back of the chisel is the registration surface and the bevel is the chip lifter. That is the whole geometry of the cut. If the bevel faces the line, the chisel wedges itself away from the line as it goes down, and the wall comes out slanted. Chopping and paring use the same edge in different ways; the difference is how the back is registered.
Slow down at about three-quarters depth. The walls are tall and a heavy blow has less support on both sides. The last 1/8 in should be taken in bites of 1/16 in or less, with the chisel checked for verticality between each one.
Through Mortises: Work Both Faces
A through mortise exposes the exit face to blowout. The fibers on the far surface are unsupported, and a chisel driven all the way from one side will push a splintered crown out of the exit before the edge severs it.
Chop halfway from the first face, turn the workpiece, and complete the cut from the second. The two cuts meet in the middle, where the bored cavity gives the waste somewhere to go. Carry the knife lines around to the second face before you start, and use them to re-register the chisel after the flip. If the workpiece cannot be turned, clamp a sacrificial backer board tight against the exit face. The method for laying out and fitting this joint is covered in through mortise and tenon.
Pare the Walls and Level the Bottom
Switch to a wide bench chisel for the walls. Place the flat back against the wall, skew the blade 5 to 10 degrees, and take shavings of 0.005 in (0.13 mm) or less. The skew reduces the effective cut width and lets the edge slice rather than dig. Work from the entry face toward the bottom.
Check the wall with a small square at several heights and along the length. The wall is correct when the square touches it over the full height and the knife line is still visible as a hairline at the surface. If the line has disappeared, you have pared past the mortise and the joint will show a gap at the shoulder.
Level the bottom with a router plane or with the chisel used bevel down and the back registered on the high spots. A bottom that slopes will hold the tenon at an angle even when the walls are square. Break the inside corners with a chisel narrower than the mortise, but do not round them; square corners resist rotation.
Fit the Tenon and Fix What Went Wrong
The tenon should enter with hand pressure and no rocking. A tenon that drops in under its own weight is loose; one that needs a mallet is tight enough to split the mortise cheek. The shoulder should close with a consistent line, and the mortise should be 1/16 to 1/8 in deeper than the tenon so excess glue has somewhere to go. If the fit is tight, pare the tenon, not the mortise. The mortise controls alignment.
Troubleshooting
Blowout on the exit face. The bite was too deep, the chisel was dull, or the cut was taken from one face only. Stop 1/8 in short, flip the work, and make the final pass from the other side with a sharp edge.
The mortise tapers from top to bottom. The chisel was levered against the wall instead of along the mortise, or the back was not held against the line on each blow. Re-register the back at the line, take lighter bites, and check verticality every 1/4 in.
The walls are shiny and burnished. A dull chisel or a bevel that is too steep is rubbing instead of cutting. Refresh the edge at the angle described in chisel bevel angles, and take thinner shavings.
Safety at the Bench
Clamp the workpiece so both hands can stay behind the cutting edge. A chisel that slips off the end of a mortise travels toward whatever is holding the work, and that is usually a hand. Use a mallet with a face wide enough to hit the handle squarely, keep the blade sharp, and wear eye protection when chopping hard, dry wood. Sharp chisels require less force, slip less often, and are safer than dull ones. A stable bench at the right height, discussed in workbench height, does as much for control as any technique.
Frequently Asked Questions
Do I need a mortise chisel, or will a bench chisel work?
A bench chisel will cut a mortise, but a mortise chisel is faster and more accurate. Its thick blade resists twisting, its back is stiff enough to lever against, and its bevel is steep enough to survive mallet blows. Use a bench chisel only for shallow mortises in softwood, and pare with it rather than chopping.
Should I drill out the waste or chop the whole mortise?
Bore whenever the mortise is deeper than about 1/2 in. The holes remove most of the mass, keep the chisel from wedging, and reduce the chance of blowing out the exit face. Reserve solid-stock chopping for very small or shallow mortises.
How deep should the mortise be?
Make the mortise 1/16 to 1/8 in deeper than the tenon is long. The extra depth gives glue a reservoir and prevents the tenon from bottoming out before the shoulder seats. For a 1-1/4 in tenon, a 1-3/8 in mortise is about right.
How do I keep the mortise square?
Check verticality after every blow, keep the back on the knife line, and take light bites. A small square at two heights shows drift before it becomes permanent.
Can I chop a mortise in plywood or MDF?
You can, but the layers and resin make the walls fragile and dull the chisel quickly. A router or hollow-chisel mortiser gives a cleaner result in sheet goods.
See Also
- Paring Cuts vs Chopping Cuts: Matching Chisel Technique to the Task
- Chisel Bevel Angles Explained: 25, 30, and 35 Degrees and When to Use Each
- Angle Gauges & Bevel Setters: The 2026 Buy-&-Use Guide for Perfect Chisel Bevels
- Chisel Choices: Decoding Western vs. Japanese Blades for Perfect Mortise-and-Tenon Joints
- 10 Top-Rated Wood Chisel Sets for Furniture Makers According to Experts