What's inside
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What a mortise gauge does
A mortise gauge marks two parallel lines for a mortise. Its two pins can also mark the matching tenon cheeks, the broad faces cut away to form the tenon. Set the pins to the mortise chisel’s width and use the same setting for both parts. That helps keep the tenon centered and sized to fit the mortise.
A centered tenon has equal shoulders on both sides of the board. If the board is 3/4 inch thick and the tenon is 1/4 inch thick, the shoulders should each be 1/4 inch. The gauge does not find the center by itself: you set it from the stock thickness, then position its fence against a face or edge.
Tools and stock to prepare
You need a mortise gauge, a rule or calipers, a square, and a sharp pencil or marking knife. For the joint itself, have the mortise chisel and a plane or chisel ready for fitting. A marking knife makes a finer, more definite line than a pencil, but use whichever marking point you can control without wandering.
A basic mortise gauge is often enough for occasional furniture work. A sliding-head gauge with two pins and a locking screw is the common style. More expensive models may have a micro-adjuster or a wheel cutter; those features can make repeat settings easier, but they are not necessary for a single centered tenon.
Start with straight, square stock. If one face is twisted or the edges are not square, the gauge can faithfully mark lines that are not centered relative to the surfaces that matter. Choose a reference face and edge, mark them, and keep the fence against those same references throughout the job.
Set the gauge for the tenon
First, choose the tenon thickness. A practical starting point is the width of the mortise chisel, though the finished mortise may vary slightly with technique and wood. For example, for a 1/4-inch mortise chisel, set the gauge pins 1/4 inch apart. A tenon that is too thick will not enter; one that is too thin leaves a loose joint.
To center that thickness on 3/4-inch stock, the space from either face to the nearest pin must be the same: (3/4 − 1/4) ÷ 2 = 1/4 inch. Set one pin 1/4 inch from the fence’s reference face, then set the second pin 1/4 inch farther over. Check the total pin spacing with a rule or calipers before tightening the lock. A small measuring error is easy to repeat across the whole joint.
If the gauge has a sliding head and you cannot directly measure each pin from the fence, use a known thickness of scrap as a setting block. Set the first pin the required distance from the fence, then measure the pin-to-pin spacing separately. Do not assume the pins are symmetrical just because the head looks centered.
Mark the tenon cheeks and shoulders
Place the board flat and put the gauge fence firmly against the marked reference face. Draw the gauge along the stock in one steady pass, keeping the fence in contact. Avoid leaning the gauge or letting it rock over rough grain. Extend the two cheek lines only as far as the tenon length requires.
For the shoulder line, measure the tenon length from the end of the board and mark it square across all four faces. A combination square or marking gauge is better for this crosswise line than trying to turn the mortise gauge around. The tenon length depends on the mating part; a 1-inch tenon is not automatically right for every joint. Leave enough material for the shoulder and avoid cutting into the shoulder line.
Before cutting, check that the cheek lines are equally distant from the two broad faces. Measure both margins at the end of the board. If one is wider, reset the gauge rather than trusting a line that only looks centered.
Transfer the setting to the mortise
Use the same gauge setting to mark the mortise on the mating piece. Keep the fence against the corresponding reference face, not an opposite face chosen at random. Mark the mortise length and position separately with a square or rule, then gauge the two sides. Keeping one reference face consistent helps the tenon align flush with the intended surface.
A mortise chisel should fit between the gauge lines with little side clearance. If the lines are narrower than the chisel, check the setting before chopping. If they are wider, the joint may be loose unless you can adjust the tenon to match. For repeated joints, verify the setting on scrap before marking every workpiece.
Choosing a marking tool
| Tool | Best use | Trade-off |
|---|---|---|
| Basic pin mortise gauge | Occasional mortise-and-tenon work | Low cost; setting small offsets takes care and a reliable rule. |
| Micro-adjust mortise gauge | Repeated work or frequent setting changes | Faster to dial in, but costs more and still needs checking. |
| Wheel marking gauge | Single lines and clean cuts across grain | Useful for shoulders, but usually does not replace a two-pin mortise gauge. |
If you need a more versatile tool for shoulders and general layout, a wheel marking gauge is worth considering. It marks one line at a time, so it is not as convenient for setting both cheeks together. For occasional work, a simple combination square and pencil can mark a centered tenon too, but it takes more layout steps and is easier to misread or shift.
Common mistakes and checks
Changing reference faces: This can move the tenon off center relative to the mating part. Mark a reference face on both pieces and keep the fence against the intended face.
Measuring from the wrong point: The pin spacing sets tenon thickness; the offset from the fence sets its position. Check both measurements independently.
Loose locking screw: A head that shifts mid-marking produces tapered or inconsistent lines. Tighten the lock and test the gauge on scrap before using it on the workpiece.
Forcing a poor fit: If the tenon binds, pare the cheeks in small, even cuts and test often. Do not widen the mortise casually; that can leave a sloppy joint or break through a mortise wall. Fit the joint to the actual marked and cut parts, not just the nominal chisel size.
Finally, make a dry fit before glue-up. The tenon should enter with firm hand pressure, seat at the shoulders, and not need hammer blows that risk splitting the stock. A careful gauge setting makes the layout repeatable, but sharp tools and a measured fitting pass are what turn the lines into a sound joint.