What's inside
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A marking gauge and a mortise gauge both scribe parallel lines, but they contact the wood differently. For laying out a tenon, that difference matters: a marking gauge has one spur, while a mortise gauge has two points set apart to mark the cheeks of a mortise. Either can mark tenon cheeks. The right choice depends on how often you cut mortise-and-tenon joints and whether you want one tool to mark both sides at once.
What each gauge does
A marking gauge has a fence that rides against a board edge and a single pin or knife that scores a line at a set distance. It is useful for setting a shoulder line, marking a cheek, or transferring a measurement parallel to an edge. You reset it to mark the second cheek.
A mortise gauge has two marking points, usually one fixed and one adjustable, plus a fence. Set the distance between the points to the mortise width, then set the fence to the distance from the reference edge. It can mark two parallel lines in one pass. On a tenon, those lines define the cheeks that fit the mortise.
Quick comparison
| Feature | Marking gauge | Mortise gauge |
|---|---|---|
| Marking points | One | Two, with adjustable spacing |
| Tenon cheek layout | Mark one cheek, reset, mark the other | Mark both cheeks in one setup |
| Best for | General parallel lines and mixed shop tasks | Repeated mortise-and-tenon work |
| Setup | Simple; one offset at a time | More settings to control |
| Common risk | Resetting the offset inaccurately | Misreading the point spacing or using the wrong pair of points |
Laying out a tenon
Start from a reliable reference face and edge. Mark the shoulder line around the stock with a knife or marking gauge, using a square to carry the line across the faces. Then set the cheek gauge from the mortise width or from the tenon thickness called for in the plan. A tenon should fit the actual mortise, not just a number on a drawing.
With a mortise gauge, adjust the two points to match the mortise width, then lock the setting. Test the spacing on scrap: the marks should correspond to the mortise walls, with a little allowance for fitting rather than a forced, friction-heavy fit. Set the fence against the reference edge and scribe the cheeks down the tenon stock. Keep the fence pressed firmly against the edge; a gap or a rocking fence shifts both lines.
With a marking gauge, set the fence to the first cheek offset and scribe it. Measure or transfer the second offset, reset the gauge, and mark the other cheek. This takes an extra setup, but it is straightforward when you have only a few tenons or when the mortise width is not known in advance. Check both offsets from the same reference face so the tenon stays centered where intended.
For either tool, use light pressure on the first pass and deepen the line with a second pass if needed. A heavy first stroke can follow grain and wander, especially with a blunt pin. Sharp points leave cleaner boundaries for sawing and chopping. Do not confuse the cheek lines with the shoulder line: cut the cheeks to those parallel marks, then work the shoulders to their separate layout line.
Which one should you buy?
If you do general joinery, make occasional tenons, or want one gauge for rabbets, board thickness, and other parallel marks, a single-point marking gauge is the sensible first purchase. It costs less, is easier to set, and does not become redundant when you are not cutting mortises. The trade-off is that matching the second cheek requires another measurement or a careful reset.
If you regularly make mortise-and-tenon joints, a mortise gauge is more convenient and helps preserve the relationship between the two cheek marks. Look for a fence that locks securely and point spacing that adjusts without slipping. Some gauges have a second set of points intended for different marking widths; confirm which points you are using before setting the distance.
A dual-purpose combination mortise and marking gauge can cover both jobs, but the extra adjustment is useful only if it is secure and repeatable. A cheaper single gauge is fine if you make a few joints a year and are willing to verify each setup. Spend more for convenient adjustment and a dependable fence, not for features you will rarely use.
Common mistakes to avoid
The most damaging error is setting the gauge from an inconsistent edge. If the stock is rough, tapered, or not square, the fence may ride differently along its length and the cheek lines will not be parallel to the intended face. Joint or plane the reference edge first, and keep the same face against the fence for every related mark.
Another failure is treating the gauge setting as proof that the tenon will fit. A mortise may vary slightly after chopping, and timber can move. Cut the tenon a little proud, test the fit, and pare high spots with a sharp chisel. If the fit is too tight, forcing it can split the mortised piece; if it is loose, the joint loses bearing area and may need to be remade.
Before cutting, inspect the layout under good light and confirm the tenon is centered or offset as planned. On a small batch, mark a test piece first. That simple check catches a wrong fence offset, reversed reference face, or gauge that slipped before the mistake is repeated across the whole job.