I second Kent's recommendation of 3/4". My 1/2"-walled boxbeam was a bit wobbly.

Ross

: Hi Francis,
: The cut you are refering to is called a rabbet cut. It is best done with a
: table saw and a Dado blade or a power router. It can be done with a
: rabeting plane, but using a plane on plywood is tuff at best. I have done
: rabbets with a circular saw and a fence, but not on something as thin as
: 1/2". You could make a series of cuts and clean the rabbet up with a
: chisel. But if you don't have the tools do with out the rabbet cut and
: just but the ends up, glue and screw it. BTW, I used 1/2" plywood and
: would go to 3/4" if I were to do it again. It's stiffer.