What's inside
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Tools and dimensions
A centered mortise is positioned equally between the two faces of a stile. The key is to mark its width from the actual timber, not from a nominal size on a plan. Stiles sold as 2-inch stock, for example, may finish at a different width after machining.
You’ll need a sharp pencil or marking knife, a rule or tape, a square, and a marking gauge. A mortise gauge, which has two adjustable pins, is useful when laying out both sides of the mortise at once. For a straightforward job, a single-pin gauge and a sharp chisel can do the same work, but you’ll make more separate settings.
A reliable marking gauge should adjust smoothly and lock without the fence shifting as you scribe. A basic wooden gauge is adequate for occasional work; a more refined gauge is easier to set repeatedly and may hold its adjustment better. Check the fence against a square before relying on it.
Choose the mortise width
For a mortise-and-tenon joint, start with the tenon thickness. A common rule of thumb is to make the tenon about one-third of the stile’s thickness, leaving roughly equal material on either side. This is a starting point, not a requirement: joint loads, timber condition, and the stile’s actual thickness all matter. Avoid leaving thin cheeks that are likely to split.
Measure the stile’s thickness with a rule or caliper, then decide the tenon thickness and mortise width. If the tenon is already cut, measure it in several places. A mortise should accept it with firm hand pressure, not require hammering it home. A fit that is too tight can split the stile, especially near an end or in dry, brittle wood.
| Layout approach | Best for | Trade-off |
|---|---|---|
| Mortise gauge with two pins | Repeated mortises or matching a known tenon | Fast and consistent, but takes care to set accurately |
| Single-pin marking gauge | One-off work and simple layouts | Inexpensive and flexible, but requires two settings |
| Rule, square, and knife | Occasional layout with no gauge available | Works well, but is slower and easier to mis-measure |
Find the centerline
Measure the stile’s width across the face where the mortise will go. Mark the halfway point at the top and bottom of the mortise area, then join those marks with a light pencil line or a knife line guided by a square. If the stile is tapered or irregular, measure the width at the mortise itself rather than assuming the center is constant along its length.
For a square-edged stile, you can also set a marking gauge to half the measured width and scribe from one edge. Turn the timber around and check from the opposite edge: the two lines should coincide. If they don’t, stop and check the measurement, the gauge setting, and whether the edges are parallel. Don’t average a bad setup into the timber.
Set the gauge to the mortise
Set a two-pin mortise gauge so the pins are separated by the intended mortise width. If matching a tenon, use the tenon itself as a reference where practical, then verify the setting on scrap. Position the fence so the pair of lines will be centered: measure from the fence to the nearer pin, then compare that distance with the space from the timber’s edge to the mortise edge you intend to mark.
With a single-pin gauge, set the fence to the distance from one edge of the stile to the first mortise edge. Scribe that side, then reset the gauge for the opposite mortise edge. You can calculate both settings from the centerline and mortise width: each mortise edge is half the mortise width from the centerline.
A mortise marking gauge is convenient for repeated joints, but it isn’t essential for a single mortise. If you buy one, make sure the pins can be adjusted to suit your intended mortise width and that the fence stays square to the stock.
Scribe and check
Hold the timber securely on a bench, with the face and edge you’re referencing clean and free of chips. Keep the gauge fence tight to the same reference edge throughout each pass. Draw it along the work with steady pressure; several light passes usually give a cleaner line than one heavy pass. A knife-edged line gives a crisp boundary for chopping, while a pencil line is easier to see but can be too wide for close-fitting work.
Before cutting, inspect the layout from both faces. The two long mortise lines should be parallel to the stile’s edge, equally spaced from the edges, and consistent with the tenon. Check the ends of the mortise with a square and mark them across the face. If the mortise is near an end, leave enough sound wood beyond it to resist splitting; the required amount depends on timber, grain, and joint forces, so don’t treat a fixed margin as a guarantee.
Common mistakes to avoid
The most common error is referencing opposite sides of a stile whose edges are not parallel. A gauge can faithfully reproduce that error. Choose one reliable reference edge and use it consistently; if the timber is noticeably out of square or tapered, true it first or lay out from a defined face and edge.
Another failure is setting the mortise from an assumed stock width. A difference of even 1/16 inch can make the joint visibly off-center or leave one cheek weak. Measure the actual workpiece and test the gauge setting on scrap of similar thickness.
Finally, don’t deepen the gauge lines by leaning hard on the tool. A wandering, ragged line is harder to chop to than a fine one. If the marks are faint, make another controlled pass. Good layout comes from a stable reference, a checked setting, and lines you can still see clearly when the chisel work begins.