Angled Mortise is Better than Angled Tenon
I’m making a chair, and the front is wider than the back. The side rails and legs are joined with mortise-and-tenons. Should I use angled mortises with straight tenons, or straight mortises with angled tenons?
Bill Eckel, Lowell, MA
An angled mortise with a straight tenon is more difficult to make (the tenon has angled shoulders), but it is a better choice for two reasons. First, because the tenon is straight, the grain runs along its length, making it strong. On angled tenons, the grain can dive toward one side, making the tenon weaker and prone to breaking. Second, after assembly a straight tenon in an angled mortise resists the stresses of racking very well, and the chair as a whole is stronger.
By the way, when building a chair with angled mortises and straight tenons, you must assemble the sides first and then attach the front and back rails. If you do it in the opposite order, you won’t be able to get the chair together because the ends of the straight tenons won’t line up with the mortise openings.