What's inside
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What a mortise gauge marks
A mortise gauge marks two parallel lines at once: the edges of the mortise on one face of the workpiece. The fence rides against the reference edge, while two pins or cutters scribe the mortise width. If the mortise is centered, the fence is set so the two lines sit an equal distance from the edges of the board or stile.
This is different from marking a single shoulder line with a standard marking gauge. A mortise gauge has an adjustable second pin, and that second pin is what makes it useful for laying out a mortise without repeated measuring. Some gauges use pointed pins; others use small cutters. Cutters usually produce cleaner lines across the grain, while pins are simpler and work well in softwood.
For occasional work, a basic wooden mortise gauge is often enough. Spend more when you need a rigid fence, fine adjustment, or reliable repeatability across several parts.
Tools and preparation
You need the mortise gauge, a rule or combination square, a sharp pencil, and a flat work surface. Use a sharp chisel to check the intended mortise width. The gauge should be sharp enough to leave a continuous line, but not so aggressive that it tears the fibers when you push it.
Before setting the gauge, identify the reference face and reference edge. Mark them with a pencil. Every measurement should come from the same faces. Switching edges halfway through the layout is a common cause of mortises that are technically centered from one side but misplaced relative to the rest of the joint.
Check the stock for twist and bow. A gauge follows the reference edge, so it can faithfully mark a badly distorted board. For a typical 3/4-inch board, a centered 1/4-inch mortise leaves 1/4 inch of material on each side. On a 1-inch stile with a 3/8-inch mortise, the remaining shoulders are 5/16 inch each.
Set the mortise gauge for a centered mortise
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Determine the mortise width. Choose a width that suits the chisel you will use. A mortise is easiest to cut when the layout matches the chisel width, or when it is a simple multiple of that width. For example, a 1/4-inch chisel suits a 1/4-inch mortise; a 3/8-inch mortise can be cut with a 1/4-inch chisel by working from both sides.
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Find the centerline. Measure the stock thickness and divide it by two. For 3/4-inch stock, the center is 3/8 inch from either face. Set a combination square to that distance and make a short mark across the end grain or on an offcut.
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Set one pin on the centerline. Hold the gauge against the reference edge and adjust one pin until it follows the center mark. Tighten the screw firmly, then test it on scrap. This establishes the center of the mortise.
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Set the second pin for the mortise width. Move the second pin one-half of the mortise width from the centerline. For a 1/4-inch mortise, set it 1/8 inch away from the center pin. The two pins will then be 1/4 inch apart and the mortise will be centered.
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Confirm the setting on scrap. Make two short test marks and measure between them. Also measure from each mark to the board edges. Do not rely only on the gauge’s graduations; inexpensive gauges can have loose fittings or inaccurate scales.
An alternative is to set the pins from the two edges. Put the fence against one edge and adjust the first pin to the required shoulder distance. Then turn the stock around or use a rule to set the second pin from the opposite edge. This method is slower but useful when the stock is not a standard thickness or when the mortise is intentionally offset.
Mark the mortise accurately
Clamp the work securely with the reference face in the same orientation used during setup. Put the gauge fence against the reference edge and push it in one smooth direction. Keep the fence flat and apply light pressure. Two or three firm passes are better than one hard pass, which can make a pin skate across the grain.
Mark the mortise on the face and extend the lines across the end or edge where the mortise terminates. For a blind mortise, mark the length with a square and knife line. A depth mark on the chisel or a piece of tape is useful, but do not treat it as a substitute for a clear end line.
When marking several identical parts, keep the same reference face and edge against the gauge. Stack or clamp parts only if their edges are flush and the gauge can ride against all of them without rocking. Otherwise, mark each part separately. Small differences in thickness can move a centered mortise noticeably.
Mortise gauge options compared
| Gauge type | Best use | Advantages | Trade-offs |
|---|---|---|---|
| Basic wooden pin gauge | Occasional joinery and softwood | Low cost, simple, easy to sharpen | Can slip if the screw or wedge is weak; pins may tear fibers |
| Wooden cutter mortise gauge | Furniture work and hardwood | Cleaner lines across the grain; good control | Costs more and needs properly sharpened cutters |
| Metal-bodied mortise gauge | Repeated work and jobsite use | Rigid, durable, often easier to adjust precisely | Heavier and usually more expensive |
A woodworking marking knife is worth adding if the gauge leaves fuzzy lines in hard maple, oak, or plywood veneer. For pine, poplar, and general site carpentry, the cheaper pin gauge is usually fine if its fence is square and its lock holds firmly.
Common failures and corrections
The mortise is off-center: the center pin was set from the wrong face, or the second pin was moved without accounting for the mortise width. Reset from a known centerline and test on scrap.
The lines are different distances from the edges: the fence was not held against the reference edge, or the stock thickness varies. Establish one reference edge and plane the stock before layout if accuracy matters.
The gauge slips during marking: tighten the locking screw, clean dust from the stem, and reduce pressure on the pins. If the fence still moves, the gauge is not dependable for repeated work.
The line is ragged: sharpen the pins or cutters and make a light first pass. On figured grain, score with a knife or mark from both directions to reduce breakout.
Once the layout is complete, place the chisel’s flat back against each line before chopping. Start with shallow cuts, then deepen the mortise in stages. That preserves the reference lines and gives the centered layout a fair chance of producing a centered joint.