Cope-and-Stick Joint

A cope-and-stick joint is how frame-and-panel doors are actually built at scale: a matched pair of router bits (or a single reversible bit set) cuts a decorative profile and a groove for the panel along the inside edge of the frame's stiles and long rails (the "stick" cut), while the coping cut, run on the ends of the connecting rails, mills a negative of that same profile so the two pieces interlock and the profile appears continuous around the whole frame once assembled.

Without the cope cut, a rail butted against a sticked stile would leave a visible, ungainly step where the decorative profile just stops -- the cope is what makes a frame-and-panel door's molded edge look like one continuous line around the opening instead of an obviously assembled frame.