Mortice Gauges: Setting Two Pins for Mortise-and-Tenon Joints That Actually Fit

You're building an oak garden gate. You cut the mortise slightly wider than the chisel because you marked the two sides separately with a single-pin gauge and they drifted half a millimetre apart. The tenon is now loose, and the joint will rack under wind load within five years. A mortice gauge sets both sides of the mortise from a single reference in one pass. The mortise comes out exactly the chisel width, the tenon fits firmly, and the joint outlives the gate. That's the whole purpose of the second pin.

What it is and when you need one

A mortice gauge is a marking gauge with two pins instead of one. Both pins sit on the same beam, but the second pin is on a smaller sliding bar (called the slip or slider) that moves independently along the beam. You set the gap between the two pins to match the width of the mortise (typically the width of your chisel), then set the fence at the offset from the timber edge. A single pass with the gauge scribes both sides of the mortise simultaneously, parallel to each other and parallel to the timber edge.

You need one whenever you're cutting a traditional mortise-and-tenon joint. That's any frame-and-panel construction, stile-and-rail door, garden gate, oak frame, traditional kitchen cabinet door, sash window repair, or repair work on antique furniture. For modern carpentry that uses dominos, biscuits, pocket screws, or dowels, you don't need a mortice gauge. For anything traditional, you do.

A standard single-pin marking gauge can mark the two sides of a mortise too, but you have to set it twice (once for each side), which means two opportunities for measurement error. The double-pin design eliminates this entirely. Both pins are referenced from the same fence face, so both lines are exactly parallel and exactly the right distance apart, every time.

Set once, scribe forever

Once a mortice gauge is set to a specific chisel width and a specific edge offset, you can use it to mark dozens of identical mortises across a project without re-measuring. Many woodworkers keep one gauge permanently set to their most-used 6mm chisel, and re-set only when the job calls for a different width.

Anatomy of a mortice gauge

Five parts make up the tool.

• Beam: the long bar (typically beechwood, rosewood, or boxwood) that slides through the fence. Holds the fixed pin at its end.
• Fence (stock): the rectangular block that presses against the timber edge. A brass face plate protects the wear surface from being chewed up by repeated use.
• Fixed pin: a sharpened steel point at the end of the beam, immovable.
• Sliding pin (slip): a second sharpened pin on a small brass bar that slides along the beam itself. The position of this pin is set independently from the fence position.
• Two thumbscrews: one locks the fence to the beam (sets the offset from the edge); the other locks the slip to the beam (sets the gap between the two pins, the mortise width). On premium gauges a third thumbscrew at the end of the beam adjusts the slip with a screw drive for fine setting.

A combination gauge adds a third pin on the back face of the beam, giving you a single-pin marking gauge on one side and a mortice gauge on the other. It's a sensible compromise if you only want one tool for both jobs.

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How to use it properly

Setting the pin gap to a chisel width

The single most important rule of mortice-and-tenon joinery: the mortise width matches the chisel width exactly. If you cut a 6mm mortise with a 5mm chisel, the joint is sloppy. If you cut a 6mm mortise with a 7mm chisel, the chisel won't fit. Set the gauge from the chisel itself, not from a tape measure.

Setting the fence offset

The fence offset determines how far in from the timber edge the mortise sits. For most mortises, this is half the timber thickness minus half the mortise width - i.e., the mortise is centred on the timber. A 6mm mortise in a 30mm-thick stile sits at 12mm from one edge (and 12mm from the other, but you only mark from one face).

Use a rule or tape measure to set the fence. Loosen the fence thumbscrew, slide the fence to the correct offset from the fixed pin (or the centre of the pin pair), and tighten. Always reference your offset from the same face of the timber for every piece in a project - this is called working from the face side, and it's what keeps assemblies square if your timber isn't perfectly square.

Scribing the mortise lines

Hold the gauge with the fence pressed firmly against the face edge of the timber. Tilt the beam slightly so the pins trail behind the direction of travel rather than digging in. Push the gauge in one smooth motion along the timber, applying steady pressure against the fence. Both pins scribe parallel lines simultaneously.

For shoulder lines (the cross-grain end of the mortise), use a knife or cutting gauge instead - pin gauges tear cross-grain fibres rather than cutting them cleanly. The mortice gauge marks only the long grain sides of the mortise; you mark the ends with a different tool.

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What the second pin gives you

The advantage isn't speed (you could mark each side with a single-pin gauge and still get there). The advantage is that both lines reference the same fence position. Whatever subtle drift exists in the fence-to-edge contact, both pins inherit it identically. The mortise width stays exactly correct even if the absolute position moves slightly.

Think of it like a shadow. Wherever the fence shadow falls, both pins follow it together. That's why an old mortice gauge in skilled hands produces consistently accurate joints even when the gauge itself isn't perfectly square. The relative measurement (the gap between the pins) is locked, regardless of any small absolute error in the fence position.

What to buy

The mortice gauge market splits into three tiers based on materials, fit, and finish.

Budget: beechwood with brass

Faithfull, Crown Hand Tools, and Stanley all produce beechwood mortice gauges in this price band, sold at Screwfix, Toolstation, and Amazon UK. The beam is straight-grained beech with a brass face on the fence, two thumbscrews, and steel pins. Out of the box the pins may need sharpening (a few strokes on a fine stone) and the slip slot may have rough edges that benefit from a light chamfer with sandpaper.

These do the job for occasional joinery work. The wood is harder than softwood pine but softer than rosewood, so the gauge wears noticeably after a few hundred uses. Tighten the thumbscrews carefully - the brass threads strip if you crank them hard.

Mid-range: rosewood and dense hardwoods

Faithfull's "Heavy Duty" rosewood mortice gauge, Crown rosewood gauges, and Stanley Sweetheart 60 Series sit in this band. The wood is denser, the brass face plate is thicker, the pin steel is harder, and the thumbscrews are more substantial. The slip mechanism is tighter, with less wobble. These are the right choice for someone planning to cut more than a handful of joints over the life of an extension or workshop fit-out.

Some mid-range gauges include a screw-drive fine adjustment on the end of the beam: turn a small thumbscrew at the beam tip and the slip moves microscopically along the beam. For setting the pin gap to within a fraction of a millimetre, this is a meaningful upgrade.

Premium: wheel-type mortice gauges

Veritas (Lee Valley, sold via specialist UK woodworking retailers like Axminster and Workshop Heaven) makes wheel-type mortice gauges that replace the two pins with two small hardened steel wheels. The wheels cut cleanly in any grain direction and don't need to be tilted to a trailing angle, which makes them more forgiving for beginners. They cost two to four times what a budget gauge does, and the difference shows in the line quality.

For someone who has used a pin gauge for years and wants to upgrade, a Veritas wheel mortice gauge is a noticeable improvement. For someone buying their first marking tool, a basic pin gauge gets the job done at a fraction of the cost.

Combination gauges

A combination gauge combines a single-pin marking gauge on one side and a mortice gauge on the other in a single tool. They cost slightly more than a basic mortice gauge but less than buying two separate tools. The compromise is that you re-set both functions every time you switch between them, so for repeated work you'd still want dedicated gauges set permanently to common dimensions.

Maintenance

The pins blunt over time. A blunt pin tears wood fibres rather than cutting cleanly, leaving a fuzzy line that's harder to follow with a chisel. Sharpen the pins occasionally with a fine stone or a few strokes on emery paper, keeping the original cone profile. Don't reduce the pin diameter - just refresh the point.

The brass face plate on the fence will burnish bright with use. Don't polish it - the natural patina helps the gauge slide smoothly along timber edges. If the face becomes sticky or sticky-feeling, a quick rub with a wax block (beeswax or paraffin wax) restores the glide.

Keep the gauge in a tool roll or a drawer, never tossed loose in a tool bag. The pin tips bend or break if the gauge is dropped pin-first, and a bent pin scribes a crooked line.

Where you'll need this

• Kitchen installation - building any traditional cabinet doors with mortice-and-tenon frames around a panel
• Decoration - fitting traditional five-panel doors or repairing existing mortice-and-tenon door frames during second-fix carpentry
• Windows and doors - sash window repairs that involve renewing tenons in stiles or rails
• Tiling (for batten frames) - building any timber frames where strength and squareness matter

For most modern construction the mortise-and-tenon joint has been replaced by easier alternatives (pocket screws, dominos, biscuits, dowels). But anywhere traditional joinery still applies - heritage doors, oak frames, sash windows, garden gates, traditional cabinet doors - the mortice gauge is the only tool that scribes both sides of the joint accurately in a single pass.

Safety

The pins on a mortice gauge are sharp enough to puncture skin if you grip the tool wrong. Hold the fence and beam, not the pins. Store the gauge with the pins facing into the tool roll or drawer so a careless reach doesn't catch a finger.

The wood is finished with shellac or oil. The dust from sanding or refinishing is no more hazardous than any fine wood dust, but use a dust mask if you're cleaning up an old gauge that may have been treated with older finishes that contained lead.