A mortice gauge is an essential marking tool for any woodworker. Once you know how to set and use one accurately, your mortice and tenon joints will be cleaner, more consistent, and far easier to cut.
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What Is a Mortice Gauge?
A mortice gauge is a precision marking tool used in woodworking to scribe two parallel lines simultaneously — essential for laying out mortice and tenon joints. Unlike a standard marking gauge, which has a single pin, a mortice gauge has two adjustable pins that can be set to match the exact width of your chisel or tenon saw. The Spear & Jackson Mortice Gauge is a quality hardwood and brass option that also doubles as a standard single-pin marking gauge.
Parts of a Mortice Gauge
Before using one, it helps to understand the key components:
- Stock (fence) — the sliding block that rides against the face of the timber
- Beam — the long bar that passes through the stock
- Fixed pin — the stationary scribing point
- Adjustable pin — the moveable point, set to define the width of the mortice
- Thumbscrew / locking nut — locks the stock in position once set
Setting Up Your Mortice Gauge
Getting the setup right is the most important step. Follow these stages:
- Set the pin spacing to match your chisel. Loosen the adjustable pin and place your mortice chisel between the two pins. Adjust until both pins just grip the sides of the blade — this ensures your mortice will be exactly the right width.
- Set the distance from the stock. Slide the stock along the beam to position the pins at the correct distance from the face of the timber. For most mortice and tenon work, this is centred on the timber thickness. Lock the thumbscrew firmly.
- Test on scrap timber. Always make a test mark on a piece of scrap before marking your workpiece. Check that the lines are centred and the spacing matches your chisel.
Marking the Mortice
Once set up, using the gauge is straightforward:
- Hold the stock firmly against the face side of the timber — always work from the same reference face to keep joints consistent.
- Tilt the gauge slightly forward so the pins trail as you push or pull along the grain.
- Apply light, even pressure and make two or three passes to deepen the lines gradually rather than forcing a single heavy stroke.
- Mark both the mortice on the leg or stile and the tenon on the rail from the same face side.
Tips for Accurate Results
- Always mark from the face side — this is the golden rule of hand-tool joinery.
- Keep the stock pressed firmly against the timber throughout the stroke to avoid drifting.
- Use a sharp pin. If your lines are ragged or torn rather than clean and crisp, the pin may need sharpening on a fine slipstone.
- For through mortices, mark both faces of the timber and chop from each side to meet in the middle.
Caring for Your Mortice Gauge
A quality mortice gauge will last a lifetime with minimal care. Wipe the beam with a little linseed oil occasionally to keep it sliding smoothly, and store it in a tool roll or box to protect the pins. Check the locking mechanism periodically — a loose thumbscrew can cause the stock to shift mid-use.
Choosing a Mortice Gauge
Look for one with a solid brass or rosewood stock, a smooth-sliding beam, and a reliable locking mechanism. A well-made gauge will hold its setting under repeated use — essential when cutting multiple identical joints. Browse our full range of Marking & Measuring Tools, including the Spear & Jackson Mortice Gauge.
For the chisels you'll be using alongside your gauge, see our Chisels & Wood Carving Tools collection, and keep everything sharp with our Tool Sharpening and Honing range.