Lucy, you've got some splinin' to do
Miter joints are some of the weakest in woodworking because of poor end-grain to end-grain gluing. That's why I always reinforce my miter joints with a spline. I used to cut the spline slots with a jig on my tablesaw, but that gets awkward with a large frame.
Recently, I began using my biscuit joiner to simplify this process. By adding an aluminum plate to the jointer's fence, as shown, the slot is automatically centered on the joint. I cut the added plate to shape using a metal-cutting blade on my bandsaw.
A couple of caveats: When cutting the 90° bird's mouth, leave at least 3⁄16 " of metal at the back of the plate for strength. And remember that you could accidentally cut into the added plate with the joiner's blade if you don't properly adjust the depth.
—Jack Williams, Elephant Butte, N.M.