This architectural moulding cap functions to trim the edge of the paneling or beadboard. It is also known as panel cap molding, paneling molding or wainscoting chair rail.
Essentially wainscoting cap molding is a sort of chairrail with a notch out in the bottom back to accept paneling or beadboard.
Notice the different sizes of notches in the wainscoting cap molding. They correspond to the various types of wainscot.