This cap moulding can cap off the top of tongue and groove panelling 36 inches above the wall.  However, its more often used as a backband for 5/8″ thick builder colonial casing. A cap mould is a generic term for a moulding that is designed to cap something off. Examples would be a wainscoting cap mould and a header cap mould. A header cap mould is similar to an architrave only you mitre the casing on all three sides of the door, then add the cap on the top portion only to get a more neo-classic look.