An intentional overhang left on the vertical stiles of a door or window sash while cutting mortises. Because mortises are often located near the ends of the boards, the pressure of a chisel or the insertion of a tight tenon can easily blow out the fragile short grain. The horn provides extra structural support during assembly and is sawn off flush only after the glue has dried.
Heard on the job
Leave a two-inch horn on all the door stiles so we don't blow out the ends when we drive the wedges in.