On wood doors that have pillars, columns, or pilasters to the
side, the cushion between the fancy capital at the top of the
pillar and the arch, pediment, or whatever.
The middle rail of a wood door, when there is an additional molding
on the upper edge. The function of the apron is to give design
emphasis and to divert water from the bottom panel. Apron rail
or molding and ornamental detail fitted on the top edge of the
locking rail offer protection to the rail and panel. Note the
way the apron is rabbeted into the rail.
Any vertical corner where two planes meet. If you run your finger
down the corner edge of a door or along the edge of a rail, you
are touching the arris.