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
Here are some supplies and tools we find essential in our everyday work around the shop. We may receive a commission from sales referred by our links; however, we have carefully selected these products for their usefulness and quality.