I've learned that it depends a lot on how you cut the mortise, and what size
tool you have to cut the mortise. If you chop the mortise by hand, and you
have a 3/8" mortise chisel, make the tenon 3/8" thick too.
I've often wondered about this too. Generally I use 1/4 inch
for 3/4 stock. But have always thought that 3/8 would be better
but then I would only have 3/16 on either side - doesn't seem
like a lot of material.
Ideally, probably a 'sloppy' 5/16 mortise with a tenon made to fit
is probably just about right.
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