Super Strong Splines for Miter Joints


Miters are not the strongest joints, but we use them all the time for joining picture frames together. One method for strengthening this connection is to add a spline, a thin sliver of wood into a slot where the two miters join together.  A common way to make these is to cut a slot about halfway through the corner after gluing the frame together and slide in the spline.

Standard spline that goes halfway through the miter.
Standard spline that goes halfway through the miter.

A better method for making splines

In this method, the picture frame and the splines are glued up all at the same time. These splines are stronger because they run all the way through the width of the miter and the grain is positioned to run perpendicular to the miter.

Making the slot-cutting jig is simple. It’s basically the opposite of my regular spline jig.

Regular slot cutting jig (left) and alternative jig.

Regular slot cutting jig (left) and alternative jig.


Making the slot-cutting jig

Glue together two scraps of plywood. One a little taller than your rip fence, and the other piece square. I added a couple screws so I wouldn’t have to wait for the glue to dry.



Next, square this assembly up by cutting off the bottom triangle of the square, shaving off a little of the rectangle at the same time.


It should stand square to the table top and fence.\



Using the jig

After cutting your picture frame pieces with their 45 degree miters. Line up one end with the triangle so that the end is flush with the table. To keep this from slipping, clamp the board to the upper part of the jig and cut out a slot.



To make the slot on the opposite end, keep the same side toward the fence and cut the slot using the other side of the triangle.


Making the splines

The grain direction of these splines if what helps give these miters their strength, You want the grain to be running perpendicular to the miters, not the same direction.  To make these, select a board that is wider than the length of the miter joint.


Then resaw a board to the thickness of your slots.


Then crosscut this thin board into the widths of your miter slots. I used a thicker board to hold down the thin splines and keep them from flying away when I cut them




Gluing up

The nice thing about these splines is that they help with the corner alignment of the picture frame and prevent them from sliding apart. Glue them into the slots and glue the miters together all at one. One the glue is dry, cut off the excess and sane the frames smooth.





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  1. Another great video, Steve. Quick question. When cutting thin strips like you did, is it safer to have the thinner part on the outside of the blade rather than against the fence? I would think having the thin piece against the fence might cause it to kick straight back. But I’m no expert.

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  2. I watched that one and was thinking about cutting the splines to thickness. To make this repeatable wouldn’t it be easy enough to make a block for the other side of the fence that you can drop into the slot int the table for repeatably on the thickness

  3. It is all good, BUT: Your blade is leaving a U-shaped bottom in the slot for the spline (see last pic), so the spline doesn’t fit flush into it. I know there are “flat bottom” blades, but they are expensive and changing a table saw blade for a few splines seems like a lot of extra effort. Is there another solution?

  4. If one uses a rip blade to cut the spline grooves, the bottom of the cut will be square rather than pointed as shown in the video. Rip blades are ground so that they cut square across the wood when cutting a partial thickness groove. Combination blades & cross cut blades generally have an Alternate bevel grind that leaves a “v” shape at the bottom of the groove.

    Thanks for all the great tips.


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