For maximum strength, try the double mortise-and-tenon joint
It provides more physical resistance to twisting, and twice the glue surface of a single mortise-and-tenon. To cut it, you need only a spiral upcut bit for your plunge router, a tablesaw saddle jig, a couple of hand chisels -- and these easy-to-follow instructions.
Sizing the joint
When you choose to use a double mortise-and-tenon, keep in mind the minimum dimensions shown right. These dimensions create tenons at least 1/4" thick with 1/4" between them, at least 1/16" tenon shoulders, and allow for a 1/4"-thick outside wall on each mortise.
Make 3/4" and deeper mortises at least 5/16" wide, as a 1/4" router bit may break when routing that deep.
Cut the mortises first
Prepare your workpieces, along with one extra for testing the mortise setup and another for testing the tenon setup. Mount an edge guide on your router and install a spiral upcut bit to match the mortise width.
Lay out the mortises; then clamp two legs together to provide a broad surface for the router to ride on. Set the edge guide, as shown in photo, and double-faced-tape stopblocks to define both ends of the mortise. For mortises near the end of a workpiece, there may not be room for a second stopblock; in this case, simply rout to the layout line.
Rout the inside mortise in each piece, reposition the workpieces so the edges that were inside are now outside, reclamp them, and rout the second mortise in each piece. Repeat these steps to rout mortises in all of the workpieces and the test piece.
Reset the edge guide to remove the waste between the mortises in the test piece only, making one large mortise. You'll use this when dialing in the setup for the tenons in the next step. With a chisel, square up the ends of all the mortises, including the test mortise.
Tenons times two
Set the tablesaw blade height to match the depth of the tenon cheek. Measure from the far face of the blade and position the rip fence to match the length of the tenon as shown in top photo. Make shoulder cuts on all four faces of the workpieces and test piece.
Build the saddle jig shown at left. Set the jig over the rip fence and position the fence to cut the wide cheek on the outside face of the test workpiece [Photo C]. Cut the two wide cheeks, flipping the test piece face-for-face between cuts, and check the fit of the tenon into the wide test mortise cut earlier [Photo D]. Make any adjustments needed to the fence position or blade height. When you achieve a satisfactory fit, cut the wide cheeks on the project parts.
Reset the rip fence and follow the same procedure to cut the narrow cheeks on the test tenon. Then cut them on the workpieces.
Transfer the locations of the inside cheeks from a mortised workpiece onto the test tenon [Photo E]. Set the rip fence to cut between these marks, and make cuts with each face against the saddle jig [Photo F], leaving the tenons a little too thick. Test the fit in a double mortise. Adjust the rip fence as needed to achieve a snug fit; then cut the inside cheeks on the project parts. Use a chisel to clean up the small ridge left between the tenons [Photo G].