Box joints, also called finger joints or comb joints, are really quite strong and have an attractive, handmade appeal which looks good on, well, everything.
And you're welcome to cut and chisel each one by hand...but, with this easy jig, you don't have to, and unless you're a purist, there's no reason not to crank them out by the dozens. This is designed for a router table, but the same approach would work for a table saw and dado blade.
**This jig is designed for 3/8" square joints, and thus uses 1/2" plywood. Adjust your measurements accordingly.**
1. Place your fence at it's furthest position from the front of the table, and add an inch to create your length. Cut two pieces of 1/2" ply at this length by 5".
2. From the same plywood, cut two more lengths, with one 1 1/4" width and one 1" width.
3. Assemble the two large pieces at ninety degrees (glue, with very tiny fastners, if any), with the vertical face on top of the base. Then add the 1" strips behind the 90-degree edge, as shown in the photo.
4. Cut a 7" inch rail that fits inside your miter gauge slot, and glue it into place.
4. Chuck a 3/8" bit (the size of your intended joints) into your router, and set the height at exactly 3/8". Like, EXACTLY. Don't mess this up.
5. Turn on the router, and push the jig into the blade around an inch...don't go as far as the support strips on the back of the joint.
6. Now, create a 3/8" square plug, about 1" in length, and glue it EXACTLY 3/8" to the left of the slot...it'll look like this:
7. To make your joints, line up the edge of your stock (which is 3/8", or 3/4", or another multiple of 3/8" in width, and in length) with the pin, and make a cut. Then, place the pin in the slot of the first cut, and cut a second slot. Then just continue as necessary.