Lon, grab a piece of plywood 3/4" thick. Put a 3/8" Dado stack in your table saw and set the fence for a 3/8" cut, 3/8" deep. You'll have to play around a bit because the 3/4" plywood is actually 23/32. What you want to arrive at is a fence and blade depth seting that puts a rabbet on the edge that is exactly 1/2 of the thickness of the plywood in both dimensions. Run the plywood on edge and cut the rabbett. Now move the fence back another 1/2 width, approx 3/8" so you cut a groove approx 3/8 from the END of the board you are joining. That's a Dado. 3/8 from the edge, 3/8 wide and 3/8 deep. The tongue you made on the EDGE of the one panel will fit perfectly into the groove you made on the inside FACE of the other. That's a housing joint. It's very strong not just from the increased gluing surface but a good mechanical joint too. The Terry Cain recommended joint is a LOCK MITER and is easy to cut with a set of lock miter cutters and a router table. NBD