Table of Contents
A through mortise and tenon is the honest version of the joint. The tenon enters one face, crosses the stile, and shows itself on the other, so there is no place to hide a gap, a blown-out exit, or a shoulder that does not seat. That visibility is why the joint is worth learning.
It is strong as well. Long glue lines on the cheeks, the mechanical stop of the shoulder, and the option to pin or wedge the tenon give it resistance to racking that a short blind tenon cannot match. It suits trestles, doors, gates, bench frames, and any piece where the structure is part of the design.
What the Through Joint Asks
A blind mortise hides its bottom. A through mortise exposes both openings, so the walls must be straight and parallel, the exit must be clean, and the tenon must be long enough to cross without bottoming. The stile also loses more material. A 1-1/2 in wide stile with a 1 in mortise leaves only 1/4 in cheeks, which is not enough for a loaded joint.
Appearance is part of the function. A through tenon can be left proud and wedged, or cut flush and drawbored. Each choice changes how the end grain reads and how the joint is locked. Decide before layout, because the tenon length and mortise depth depend on it.
Strength comes from the glue on the cheeks, the shoulder bearing on the stile, and the pin or wedge bearing on the tenon. A well-cut joint uses all three; a joint that relies on glue alone is less tolerant of a loose fit.
Proportions and Layout
The tenon thickness should be about one-third of the rail thickness. In 3/4 in stock that means 1/4 in; in 7/8 in stock, 5/16 in. Thicker tenons weaken the mortise walls; thinner ones are fragile and give a small glue area.
The tenon width is usually half to two-thirds of the rail width, leaving at least 1/4 in shoulders on each side. On a wide rail, split the tenon into two or three smaller ones to reduce cupping and preserve more of the stile. A haunch, a small tongue above the main tenon, fills a panel groove and keeps the joint airtight.
Set the mortise gauge once, from the tenon stock, and use the same setting for the mortise and the tenon. Mark the mortise on both faces because you will chop from both sides, and carry the tenon cheeks down the rail. Every mark should register from the face side and face edge.
Mark the shoulder line around all four faces of the rail with a knife and square. It is the boundary between the tenon and the rail, and it must be continuous. A line that runs out by 1/32 in at one corner will show as a gap when the joint is clamped, no matter how well the tenon fits.
Chopping the Mortise From Both Faces
Knife the perimeter on both faces; the line severs the surface fibers and gives the chisel a place to stop. If the mortise is large, bore out most of the waste with a brace and bit, keeping the holes 1/16 in inside the lines. Boring reduces the chopping and the chance of blowing out the exit.
Chop from the face side first, working halfway through the stile. Stand the chisel vertically, flat back toward the waste, and strike with a mallet. Take light cuts near the knife lines and heavier cuts in the middle. Clamp the stile and keep the hand steadying it out of the chisel's path. Turn the stile and chop from the exit face to meet the first cut; the meeting point is hidden by the tenon. The full mortise-chopping sequence covers stance, chisel rotation, and paring.
Leave the knife line standing until the final paring pass. Pare the walls with a wide chisel, working from the ends toward the middle, and check with a square and straightedge. A slight hollow is acceptable because the tenon bridges it, but a bulge will bind. Undercutting makes fitting easier but reduces the glue area and can show as a shadow on the exit face.
For the cleanest exit, score the perimeter, clamp a sacrificial backing board over the face, and chop toward the center. The backing board supports the fibers as the chisel breaks through. If the exit face is veneered or figured, take the last 1/8 in with a router plane or a sharp paring chisel.
Sawing the Tenon
Mark the tenon from the mortise, not the ruler. A gauge set to a nominal 1/4 in may be 0.010 in off from the chisel that cut the mortise. Use the same gauge setting, or measure both ends and split the difference.
Saw the cheeks first. Hold the rail in the vise with the end grain up, start the saw on the end grain at the cheek line, and saw down to the shoulder line. Keep the plate square to the face and check the reflection of the board in the saw plate. Then saw the shoulders across the grain with a fine backsaw and a knife line, on the waste side, leaving the line for the shoulder plane.
The shoulder is the part everyone sees. A shoulder 1/64 in out of square shows as a dark gap under the rail, and no clamp will close it without crushing the wood. Trim it with a shoulder plane or rabbet plane, taking light cuts and checking against a square. A slight undercut at the inside corner, about 1/64 in, lets the shoulder seat without a glue fillet holding it proud. Do not undercut the outer edge; that line must close.
If the tenon passes through the stile, mark the exit length by holding the rail against the stile in its final position. Add 1/16 in for planing flush, or leave it proud as the design calls for. A tenon that is too long bottoms out before the shoulder seats, which is a common cause of a gappy through joint.
Fitting and Clean Shoulders
A through tenon should enter with hand pressure. If you need a mallet, something is tight, and forcing it compresses fibers and leaves a joint that loosens later. Test with the rail square to the stile, then rotate it slightly; a tenon that binds only when rotated has a twisted cheek.
Check the shoulder gap with a light behind the joint. A gap at the top or bottom means the shoulder is out of square or the tenon is not centered. A gap at the inside edge means the mortise wall is proud or the tenon is thick at the shoulder. Pare the tenon rather than the mortise; the mortise is harder to correct and its walls are the reference.
If the tenon rocks, the cheeks are not parallel. This usually comes from sawing one cheek and then the other without checking thickness at both ends. Calipers set to the mortise width find the high spot in seconds. Pare it, re-check, and repeat until the tenon slides in with light friction.
Once the joint seats, mark the tenon's position on the exit face and decide how to lock it. A dry fit with the stile and rail clamped square is the last chance to catch a twist. Use a paring cut for the final passes; chopping now removes material too quickly.
Drawboring, Wedging, and Glue-Up
Drawboring suits a joint assembled dry or glued in a humid shop. Offset the tenon hole about 1/16 in toward the shoulder, drill the stile straight through, and drive a slightly tapered peg. The offset pulls the shoulder tight, and the joint stays tight if the glue creeps. The drawboring sequence depends on drilling while the joint is clamped, so mark the holes before disassembly.
Wedging suits a tenon left proud. Cut one or two kerfs in the tenon end, aligned with the grain, and drive wedges from the exit face. The wedges spread the tenon against the mortise walls. Stop the kerf about 1/4 in short of the shoulder; a kerf that runs too far splits the tenon there. The wedged joint is decorative as well as structural, so the wedge material and the chamfer matter as much as the fit.
Do not drawbore and wedge the same joint unless the design accounts for both. The drawbore pulls the tenon toward the shoulder; the wedge pushes it back toward the exit face. Together they can leave the shoulder open or split the tenon. Choose one method, or use a pin as decoration after the wedge seats.
Glue the mortise walls and tenon cheeks with a thin, even film. A through mortise vents air as the tenon enters, so hydraulic lock is less of a problem than with a blind joint, but too much glue still has to go somewhere. Glue the shoulder lightly as well; it is long grain and adds strength.
Clamp across the stile, not along the rail, so the pressure closes the shoulder. Use cauls to protect the faces and spread the pressure. Check square and wind before the glue sets. A bridle joint is a useful comparison: it is more open, easier to check, and sometimes better when the end grain is meant to show. After curing, cut the tenon flush and plane the end grain, then chamfer 1/16 in to prevent splintering.
Frequently Asked Questions
How thick should a through tenon be?
About one-third of the rail thickness: 1/4 in for 3/4 in stock, 5/16 in for 7/8 in. Also check the stile; less than 1/4 in of cheek on each side is too little.
Should the tenon go all the way through?
Yes. It should cross the stile and finish flush or slightly proud. If it stops inside, the joint is a blind mortise and tenon.
How do I stop tearout on the exit face?
Score the perimeter, clamp a backing board over the exit face, and chop from both sides toward the center. Take the final 1/8 in with a paring chisel. The backing board is the best defense in soft or figured wood.
What is the difference between drawboring and wedging?
Drawboring pulls the joint together with an offset peg; wedging spreads the tenon with wedges driven into kerfs. They pull in different directions, so they are usually alternatives.
Why does my shoulder gap when I clamp the joint?
The tenon is probably bottoming, or the shoulder is out of square. Check the tenon length first, then the shoulder. A tenon 1/16 in too long holds the shoulder open no matter how much pressure you apply.
See Also
- Dovetail Layout: Angles, Baselines, and a Repeatable Workflow
- How to Read Wood Grain: Selecting and Machining Boards for Every Project
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- Stop Warped Miters with the 10 Best Sliding T-Bevels in 2026
- 10 Best Cranked Chisels for Flush Trimming in 2026