Table of Contents
A mortise gauge is a marking gauge with two spurs instead of one. That single change removes a common source of error in hand-cut mortise and tenon joints: setting the same gauge twice and getting two slightly different lines. With both spurs on one beam, the width of the mortise is fixed by the tool rather than by your memory of a rule reading.
The tool only helps if the spurs are set to the chisel that will cut the hole and the fence registers on the face the tenon will also use. Get those two things right and a mortise becomes repeatable. Get them wrong and the gauge produces two confident, consistent, wrong lines.
Why Two Spurs Beat Two Settings
With a single marking gauge, cutting a mortise means setting the fence for the near wall, marking every part, re-setting the fence for the far wall, and marking again. Each setting is a chance to misread the rule by half a graduation, and the two errors add directly to the width of the mortise. By the third part in a run, the setting has usually moved.
A twin-spur gauge removes the second setting. The spur spacing is locked once, and every mortise in the run comes out the same width. Because both lines are cut from the same fence registration, the walls are also parallel to the reference face by construction. That is the real value: not speed, but the fact that the two walls cannot disagree with each other.
The same setting can mark the tenon cheeks, which is the quiet advantage of the tool. Mark the mortise, then use the identical gauge setting on the tenon stock, and the fit is built into the layout instead of being chased with a chisel.
How the Twin-Spur Head Works
The head has a fixed spur near the fence and a sliding spur on a carrier that moves along the beam. The carrier locks with a screw or a wedge, the fence locks separately, and the two adjustments are independent. That independence means two locks can slip.
Check that both spurs protrude the same amount, roughly 1/16 inch (1.5 mm). A short spur does not reach the bottom of the line, so the beam rocks and the line wanders. The points should be filed to a short knife edge, not a needle. On most gauges the flat face of each spur should face the waste between them, so the inside wall of each line is the clean, square side the chisel registers against.
Tighten the spur carrier first, then the fence, then re-check both. A wedge lock can loosen during a long run, so check it at each part. If the fence moves while the carrier stays put, the mortise shifts off center without changing width, which is a distinctive and confusing failure.
Setting the Spurs to the Chisel
Measure the chisel's actual cutting edge with calipers. Nominal sizes lie. A 1/4-inch mortise chisel often measures between 0.240 and 0.250 inch, and a bevel-edge chisel can be slightly wider at the edge than the number on the handle. Set the spurs to the measured width, not the nominal one.
Then close the sliding spur by about 0.005 inch (0.13 mm), roughly the thickness of a hair. The mortise ends up a touch narrower than the chisel, so the chisel's corners shave the walls clean as it goes down. Never set the spurs wider than the chisel: the tool will not reach the corners, and the walls will be left stepped. A 1/4-inch chisel that measures 0.248 inch gets a spur setting of about 0.243 inch; a 3/8-inch chisel measuring 0.372 inch gets about 0.367 inch.
A registered mortise chisel has straight sides and a thick blade that stays in the hole without deflecting. A bevel-edge bench chisel can chop a shallow mortise, but the bevels push it toward one wall and the mortise bellies. If you are using a bench chisel anyway, choose the widest bevel angle you have, since a steeper bevel deflects less; the tradeoff between 25, 30, and 35 degrees is covered in chisel bevel angles explained.
Centering the Mortise
For a centered mortise, the fence setting is half the stock thickness minus half the mortise width. In 3/4-inch stock with a 1/4-inch mortise, that is (0.750 - 0.250) / 2 = 0.250 inch from the face to the near spur. Set the fence with a rule, then check the result by measuring from both edges of the stock to the spurs. If the two numbers do not match, the stock is not the thickness you assumed or the fence moved.
A faster method that self-corrects for odd stock is to mark the centerline with a square and knife, set the spurs to the mortise width, then slide the fence until the spurs straddle the centerline equally. Check the result with a rule before cutting. Mark the centerline from both edges, splitting the difference if the stock is uneven.
Offset mortises are deliberate, not accidental. For a haunched or shouldered joint, the fence setting is the centered value plus the offset, measured from the same face side. Write the offset on a story stick rather than carrying it in your head. A door stile or a leg with a deliberate offset is exactly the kind of part where a remembered dimension becomes a re-cut rail. The stick method is described in story sticks for layout.
Gauge both walls from the face side, never one from each face, unless the two faces are known parallel. If the stock is thicker at one end, the fence follows the face and the mortise stays parallel to that face, which is what the tenon needs. Gauging the far wall from the opposite face guarantees a tapered mortise on a tapered board.
Cutting the Lines and Chopping to Them
The gauge marks the walls; it does not mark the ends. Knife the ends with a square, working from the same face and edge references. A mortise with clean walls and a ragged end is still a bad mortise, because the tenon shoulder has nothing square to seat against. Carry the end lines around the arris with the square so the marks survive the first few chops.
Clamp the work and keep both hands behind the chisel edge; a mortise chisel that slips can do real damage. Chop the ends first, remove the middle, then work back toward the gauge lines. Leave the lines visible until the final pass, and pare the last few thousandths with the chisel back registered against the wall. The sequence matters: if you chop straight down the line at the start, the line disappears and you have no reference for the rest of the hole. The full sequence is in chopping a mortise by hand.
For a through mortise, gauge and chop from the face, then turn the work and finish from the other side to avoid blowing out the exit. A backing board clamped over the exit does the same job with less risk. If the joint will be pinned, drill for the drawbore after the dry fit, not before; the pin hole positions are covered in drawboring pinned mortise and tenon joints.
Troubleshooting the Mortise That Will Not Fit
The tenon is tight at the ends and loose in the middle. The chisel tilted at the start or the ends were undercut, so the mortise bellies. Check the walls with a square and a straightedge, then pare the ends back to the line. The cause is usually chopping the ends before establishing the middle.
The tenon is tight all around. The spur setting is narrower than the chisel, or the chisel is wider than the mortise. Re-measure the chisel, reset the spurs to the measured width, and re-cut a test mortise in scrap before touching the finished part.
The mortise is off center. The fence was registered off the wrong face, or the arithmetic used a nominal thickness instead of the actual one. Measure from both edges, and reset the fence from the face side. If the error is small, offset the tenon to match rather than widening the mortise; a widened mortise loses the wall the chisel needs.
The walls are not parallel. The face is not flat, or the fence was loose. Check the face with a straightedge, then check the fence lock. A crowned face makes the gauge ride in and out, and the resulting mortise is tapered in a way no amount of paring will fix.
The gauge line disappears after the first chop. The line was too shallow, or the chisel was allowed to ride over it. Take a second gauge pass to 1/32 inch, and chop with the chisel's back inside the waste until the last paring pass.
Frequently Asked Questions
Can a mortise gauge be used as a regular marking gauge?
Yes. Lock the sliding spur against the fixed one or ignore it, and use the fence-to-spur distance like a single-spur gauge. A dedicated gauge is easier to set for single lines. The single-spur technique is covered in marking gauge setup and technique.
Should the mortise be the same width as the tenon?
The mortise is cut to the chisel, and the tenon is cut to the mortise. Aim for a light push fit with no side-to-side play, and leave a few thousandths of clearance on the cheeks for glue.
How deep should a mortise be?
A common rule is a depth equal to the tenon width, up to about 1-1/4 inch (32 mm) in 3/4-inch stock. Deeper is not automatically stronger: leverage on the mortise walls grows with depth. Stop at about two-thirds of the stile width so the mortise does not weaken the outside face.
Do I need a dedicated mortise chisel?
For anything deeper than about 1/2 inch, yes. A mortise chisel's straight sides and thick blade keep the walls true and resist deflection. A bevel-edge chisel will work for shallow hinge mortises, but it will steer toward the bevel side in a deep hole.
Why does my mortise gauge drift on long stock?
The fence is riding a high spot, or the spur is following the grain. Gauge from each end toward the middle on long parts, and check the face for flatness first. A gauge line that drifts on a flat face is usually a dull spur or a loose carrier, not a technique problem.
See Also
- Marking Gauges: Setup, Technique, and the Lines That Make Joinery Fit
- 2026's Top 10 Laser Measures for Large Workshop Layouts
- The Science Behind Straight Edges: Why a $30 Rule Can Outperform a $300 One
- The 10 Ultimate Contour Gauges for Replicating Molding Profiles in 2026
- 10 Premium Marking Gauges Worth the Splurge for Hand-Tool Purists