A traditional miter joint creates a clean corner because the end grain is concealed inside the joint. But relying on the mitered surfaces alone doesn't provide much glue area.
A mitered bridle joint adds interlocking joinery to the corner while preserving the clean appearance of the miter.
The joint is similar to an open mortise-and-tenon joint. One workpiece has a through slot, while its mating piece has a matching tenon. The shoulders of the two pieces are cut at 45° so they come together to form the miter.
The result is an attractive frame joint with considerably more glue surface than a basic miter.
How a Mitered Bridle Joint Works
At first glance, the assembled corner looks much like a conventional miter. The difference is hidden inside.
One frame member receives a centered slot in its end. The mating piece has a tenon sized to fit that slot. Instead of square shoulders, however, the shoulders are mitered at 45°.
The key to making the joint successfully is getting a snug fit between the slot and tenon.
For that reason, I like to cut the slot first. Once its width is established, you can sneak up on the tenon thickness until it fits.
Size the Slot for the Workpiece
A good starting point is to make the slot approximately one-third the thickness of the workpiece.
For example, when working with 3⁄4"-thick stock, a 1⁄4"-wide slot divides the end neatly into thirds.
For slots 1⁄4" or wider, a dado blade works well. A narrower slot for a 1⁄8"-thick tenon can be cut with a single saw blade.
Cut the Bridle Joint Slot
Because the workpiece needs to pass over the blade while standing on end, it needs solid support. A tenoning jig provides that support and keeps the workpiece securely positioned during the cut.
You can use either a commercial jig or a shop-made version such as our Adjustable Tenoning Jig.
RELATED PROJECT PLAN: Adjustable Tenon Jig
Set the rip fence so the distance between the blade and tenoning jig positions the slot approximately one-third of the workpiece thickness from the face.
Clamp the workpiece securely in the tenoning jig and make the first cut.
Then flip the workpiece and make the second cut. Referencing the slot from both faces automatically centers it on the thickness of the workpiece.
Woodsmith Tip: Cut the Slot First. Don't try to make the slot fit a predetermined tenon. Cut the slot first and consider its finished width your reference dimension. You can then gradually adjust the mating tenon until you get the fit you want.
Cut the 45° Miter
With the slot complete, the next step is creating the mitered portion of the joint.
Use a combination square to lay out the 45° cut. Then set the table saw miter gauge to 45° and cut the miter to the layout line.
Accuracy matters here. The tenon provides strength, but the visible mitered shoulders still need to meet tightly for the joint to look right.
RELATED ARTICLE: How to Cut Perfect Miters on a Table Saw
Cut the Matching Tenon
Now you can use the completed slot as a reference for making the mating tenon.
Unlike the slotted piece, the tenon is cut with the workpiece lying flat on the table saw.
Install a dado blade and use the slotted workpiece to help establish the blade height. Set the blade slightly below the opening of the slot initially.
Then make your first cuts on a piece of scrap.
Gradually raise the blade and test the resulting tenon in the slot. The idea is to sneak up on the fit rather than accidentally making the tenon too thin.
Once the blade height is dialed in, you're ready to cut the project pieces.
Form the Mitered Tenon Shoulders
Set the miter gauge to 45° and mark the shoulder location on the workpiece.
Align the layout line with the saw blade. The rip fence can be used as a stop to establish the first cut.
Make the shoulder cut, then remove the remaining waste with a series of passes toward the end of the workpiece. This forms one cheek of the tenon.
Flip the workpiece, adjust the miter gauge for the opposite direction, and repeat the process on the other face.
The completed tenon should remain centered on the thickness of the workpiece and slide snugly into the slot.
Fine-Tune the Fit
A dado blade can leave small saw marks on the tenon cheeks, so the joint may need a little cleanup before assembly.
A sanding block works well for smoothing the surfaces. If the tenon is still noticeably too thick, a shoulder plane or chisel can remove material more quickly.
Work carefully and remove only a small amount at a time.
It's also important to remove material evenly from both faces of the tenon. Taking too much from only one cheek moves the tenon off center, which can prevent the mitered shoulders from aligning properly.
Test the fit frequently.
You want the tenon to slide completely into the slot with the mitered shoulders meeting tightly.
Glue and Clamp the Mitered Bridle Joint
Once you're satisfied with the dry fit, the frame can be glued together.
A band clamp is particularly useful for a four-sided mitered assembly because it pulls all four corners together simultaneously.
Clamping blocks and additional clamps can be added where necessary to apply pressure directly across individual joints.
Before leaving the assembly to dry, check that the miters are closed and that the frame remains square.
RELATED ARTICLE: Mitered Frame Clamping Tips & Tricks
Double Bridle Miter Joint
There's another variation worth knowing about: the double bridle miter joint.
Instead of one frame member containing the slot and the other containing the tenon, each half of a double bridle joint has both a slot and a tenon.
The two pieces are identical. Flip one member over and its offset tenon aligns with the slot in the mating piece.
That symmetry can simplify layout and machining while also producing a similar appearance along both edges of the frame.
The double bridle miter has traditionally been useful for artists' canvas frames, but the same joint can be used for other frame projects where you want the appearance and strength of this interlocking corner.
Why Use a Mitered Bridle Joint?
The appeal of this joint is that it combines two useful characteristics.
You get the clean appearance of a mitered corner along with the additional glue surface provided by the slot and tenon.
And despite its appearance, you don't need complicated machinery to make it. A table saw, miter gauge, dado blade or standard blade, and a tenoning jig are enough.
The sequence is the important part: Cut the slot first, use it to establish the tenon thickness, sneak up on the fit, and then fine-tune the cheeks until the mitered shoulders close tightly.
Get those steps right and you'll have a distinctive frame joint that's both attractive and strong.
