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A tenon should be one-third the thickness of a rail—at least, that’s what an old rule of thumb recommends. A 3/4″ board should have 1/4″ tenons, for example. But what about a 7/8″ board? What’s one-third of that?
I use my dial caliper to help solve this type of problem. I usually make a tenon the same size as one of my mortising bits, not some in-between fraction. So, for a 7/8″ thick board, should the tenon be 1/4″ or 5/16″ thick? If I could “see” the tenon on the board, I’d know which one to pick. That’s easy: I just place the dial caliper on the board and open it to 1/4″. Now I can instantly tell what a 1/4″ tenon will look like—and its shoulders, too. Then I open the caliper to 5/16″, make a mental note of what that tenon looks like and decide. –John English
