What's inside
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What a mortise gauge does
A mortise gauge marks two parallel lines for a mortise, while a single spur or pin marks a reference line for a tenon shoulder or the edge of a board. The twin lines define the mortise width; the single line helps keep the tenon centered or aligned. It is a simple tool, but a poor fence, loose head, or slipping spur can turn a tidy layout into a loose joint.
For most furniture work, a mortise gauge with two adjustable pins and a separate single pin covers the useful jobs. The key buying question is not how many parts it has, but whether the head locks squarely, the fence registers reliably against an edge, and the pins can be set to match your chisel.
Types and trade-offs
| Type | Best for | Trade-off |
|---|---|---|
| Traditional wooden gauge | General bench work and light, controlled marking | Comfortable and easy to adjust, but wood can swell, shrink, or wear around the lock |
| Metal gauge | Frequent use or a preference for a rigid, durable body | Often heavier; a small, poorly shaped fence can still be awkward to register |
| Combination mortise gauge | One tool for twin mortise lines and a single reference line | Convenient, but extra adjustments can add play if the mechanism is poorly made |
| Wheel-marking gauge | Clean single lines across grain and along an edge | A wheel is not a substitute for two correctly spaced mortise pins |
A traditional wooden gauge is not automatically inaccurate. A well-fitted head and sound lock matter more than material. For occasional mortises, a basic wooden mortise gauge is often enough. Pay more for a better-made tool if you mark joints regularly or need repeatable settings across a batch.
Features worth checking before you buy
Test the head before buying if you can. It should slide along the beam without rocking noticeably, then lock firmly without shifting its position. Tighten and loosen the lock several times: a screw that drags the head out of square or needs excessive force is a nuisance you will meet on every setup.
Look for a fence with a broad, flat face and enough beam length for the work you do. A fence that is too small is easy to tip, especially on narrow stock. Check that the twin pins adjust independently and stay put when locked. Replaceable pins are useful if one gets damaged, but they are not worth much if replacements are hard to find.
Some gauges use sharp pins; others use small cutters. Sharp pins suit general layout and are straightforward to touch up. Cutters can score a cleaner line, particularly across the grain, but may need more careful maintenance. In either case, dull or burred points tear fibers and make the line harder to follow with a chisel.
If you already own a marking gauge, consider whether a dedicated twin-pin tool is worthwhile. A combination mortise and marking gauge saves bench space, while two separate gauges can be quicker when switching between a mortise width and a shoulder reference without resetting.
Set the gauge to the chisel, not the drawing
For a chopped mortise, set the twin pins to the width of the mortise chisel you will use. A nominal 6 mm chisel should produce a mortise that accepts that chisel without forcing it sideways. Measure the actual blade if the fit matters: tools sold at nominal sizes can differ slightly, and older chisels may have been ground unevenly.
On a combination gauge, set the two pins first, then lock them and check the distance at the tips. Set the single pin separately for your reference mark. Make a test on scrap with the same species and grain direction. If the gauge leaves two ragged or wandering lines, sharpen the points or adjust your pressure before marking the workpiece.
Marking mortises and tenons without drift
Set the fence from a consistent face or edge, not from whichever surface happens to be nearest. If the mortise is meant to sit 10 mm from a face, set the gauge once and mark every matching part from that same reference face. Flipping a board or alternating between faces can shift the joint by twice the error in your setup.
Keep the fence tight to the edge and draw the gauge toward you with steady pressure. One or two light passes are safer than forcing a deep score in one pass. A hard shove can make the fence ride up over a small dent or grain ridge, moving both lines together. Check that the marks remain parallel to the edge at both ends before chopping.
For a matching tenon, transfer the mortise position from the same reference face and mark the cheeks to suit the actual mortise. Do not assume the nominal chisel width guarantees a perfect fit: a narrow tenon can be pared, but an oversized one requires extra work in the mortise. Leave a little room for fitting rather than trying to force a tight joint together.
Common problems and useful alternatives
If the two lines are unevenly spaced, check that the pins are parallel and locked securely. If both lines wander, the fence is probably lifting or the edge is not straight. If the marks are hard to see, use a sharp pencil to identify the waste side, or deepen the score carefully with a marking knife. Avoid leaning hard on the gauge: a deep groove is difficult to correct if the setting was wrong.
A cheaper gauge is fine for occasional work, softwood, or rough joinery where you will verify the layout before cutting. Spend more when you need consistent settings for several matching joints, work in dense hardwood, or find that a loose head keeps changing the spacing. For fine single-line layout alongside the mortise work, a separate marking gauge can be a sensible addition, but it will not replace the twin pins needed to lay out a mortise.