The easiest mortise-and-tenon joint ever

The mortise-and-tenon joint requires two bits: a 1⁄4" upcut spiral bit for the mortise, and to cut the tenon, a straight bit whose diameter exceeds the length of the tenon.

The router table works well for mortises up to ¹⁄₄ " wide. With mortises larger than that, I find I have better control by plunging the bit into the work with a plunge router rather than plunging the work onto the bit.

Before heading to the router table, lay out the mortise locations on the workpieces, but mark on the faces opposite where the mortises will go. This way you can see the marks as you rout the mortised face.

Set height of the spiral uncut bit

Set the height of the spiral upcut bit to ⁹⁄₁₆ " (to accept a ¹⁄₂ "-long tenon). Position the fence to locate the mortise on the workpiece. Slide a square piece of scrap against the flutes of the router bit. Transfer both edges of the bit onto the fence. The left line is your start line. The right line is your stop line. Lock the fence in place—from here out it must not move.
Clamp a 1⁄4" plywood skin to the router table

Clamp to the router table a ¹⁄₄ " plywood skin with a cutout for the bit. (The skin reduces the depth of cut for the first pass so you need not adjust the router.) Align the line for the left end of the mortise with the start line. Plunge the workpiece onto the spinning router bit while also maintaining pressure against the fence.
Remove temporary plywood spacer

Move the workpiece forward until the right mortise mark aligns with the stop line. Shut off the router, wait for the bit to stop spinning, and then lift the workpiece off the table. Remove the temporary plywood spacer and repeat the process.
Lay out the tenon

Lay out a tenon on the end of a test rail the same thickness as your project pieces. Set the straight bit so the top of its cutting edges align with the bottom of the tenon layout line. Position the fence so the bit cuts a ¹⁄₂ "-long tenon.
Complete the tenon

Back the test piece with a scrap and make a pass on each face to produce the tenon. Check the fit. If the tenon is too thick, raise the bit slightly. Remember, because you cut both faces, raise the bit by half the amount you need to remove.
Push tenon into the mortise

When the tenon fits correctly, you should be able to push it into the mortise with hand pressure or gentle taps from a mallet. With the tenon partially seated in the mortise, there should be enough friction that, if you lift the tenoned workpiece, the mating piece comes with it.
Complete the tenon

Complete the tenon by standing the rail on edge and cutting the top and bottom shoulders. A 4×4 makes a great backer block for this cut. Note that the two faces of the fence touch to prevent the workpiece from falling into a gap between them.
Clamp the tenoned workpiece in a vise

Finally, clamp the tenoned workpiece in a vise and use a bench chisel or rasp to round the tenon’s corners to match the mortise. You’re done!