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A mortise gauge and a marking gauge both scribe lines parallel to an edge, but they do different jobs. A marking gauge has one pin; a mortise gauge has two points that mark the sides of a mortise. For laying out a tenon, the best choice depends on whether you need to mark its cheeks in one setup or are willing to set and reset a single cutter.
What each gauge does
A marking gauge uses a fence that rides against the workpiece edge and a single spur, pin, or small knife that scratches a line at a set distance. It is useful for shoulder lines, baseline marks, and any layout that needs one parallel line at a time.
A mortise gauge has a fence and two marking points. On an adjustable model, one point stays in place while the other moves to set the distance between them. That spacing can match the width of a chisel, letting you mark both sides of a mortise—or the two cheeks of a tenon—in one pass.
Some gauges use a fixed pair of points, and combination gauges may include an extra pin that can be parked or adjusted. Check the mechanism before buying: “mortise gauge” does not always mean every point can be positioned independently.
Which is better for tenon layout?
For repeated tenons, a mortise gauge is usually faster and more consistent. Set the points to the intended tenon thickness, register the fence on a reference face, and scribe both cheek lines. The same spacing can then mark the matching mortises, provided you keep the reference face and gauge setting consistent.
A marking gauge works well for a one-off tenon. Set it to one cheek distance, mark one side, then change the setting for the other. This takes longer and creates another opportunity for error, but it avoids buying a specialized gauge if you already own a good marking gauge.
Neither tool lays out every tenon line by itself. You still need shoulder lines across the face and edges, usually marked with a square and marking knife or gauge. A mortise gauge is for the parallel cheek lines; it does not automatically establish tenon length or shoulder position.
Quick comparison
| Tool | Best use | Advantage | Trade-off |
|---|---|---|---|
| Marking gauge | Occasional or simple tenons | Lower cost; useful for many general layout jobs | Cheeks require separate settings or repeated passes |
| Mortise gauge | Repeated mortise-and-tenon work | Marks two parallel lines at a matched spacing | More parts to adjust; can be unnecessary for one-off work |
| Combination gauge | Shop work that mixes both tasks | Can provide single-line and paired-line marking | Extra adjustment points need careful checking |
Set the gauge for the joint
Start with the chisel or mortise width you intend to use, not a guess based on the timber. For a hand-cut mortise, a common starting point is a chisel around one-third of the stock thickness. In 3/4-inch (19 mm) stock, that suggests a chisel close to 1/4 inch (6 mm), though the joint and load may call for a different size. The tenon thickness should suit the mortise, with enough clearance for a controlled fit rather than forcing the joint together.
Set the gauge points to the actual chisel width, then test the spacing on scrap. Place the fence against the same reference face you will use on the real parts. If the stock is 3/4 inch thick and the tenon is centered, the cheek lines should be about 1/4 inch from each face for a 1/4-inch tenon. If you want an offset tenon, set the fence from the chosen reference face and measure the other cheek separately.
Make a light first pass, then deepen the line with a second pass if needed. Heavy pressure can make the fence drift or tear the grain, especially with a blunt pin or across figured wood. A clean, visible line is the goal; a deep groove can weaken a narrow tenon and make it harder to pare accurately.
Common mistakes and failure modes
The most frequent layout error is changing reference faces between parts. Marking the tenon from one face and the mortise from the opposite face can produce a joint that is off-center or has uneven walls. Mark a reference face on every part, and keep the gauge fence against it throughout the layout.
Another problem is assuming the gauge setting has not shifted. Locking screws can loosen as you work, and inexpensive mechanisms may have play. After setting the points, scribe a short line on scrap and measure the spacing with calipers or compare it directly to the chisel. Recheck after a few parts or any adjustment.
On softwood, a sharp spur can follow the grain instead of the fence line. Use a light touch and make a short test mark. If the line wanders, try a knife-point gauge or a marking knife against a straightedge for critical sections. On end grain, expect a rougher scratch and rely on a square and careful knife work for the shoulder.
What to buy
If you mostly mark shelf dados, shoulders, and occasional tenons, a reliable single-point tool is enough. A woodworking marking gauge is the economical choice, and it remains useful even if you later add a mortise gauge.
If you cut several matching joints at a time, look for an adjustable adjustable mortise gauge with a fence that locks firmly and points that can be set without slipping. A larger fence is easier to keep seated on broad stock, while a compact one is handier on narrow parts. The cheapest models can do the job, but test their adjustment for play before trusting them with a batch of parts.
For a first tool, choose based on how often you make paired lines, not on the name alone. One or two tenons a year rarely justify a dedicated gauge; repeated furniture work is where the time saved by setting both cheek lines together becomes worthwhile.