Eastern style wood double-hungs have a facing board, - but no blindstop (like a Western casing). To avoid an unsightly “overlap” installation, the (HOL) unit is mounted inside the 3/4” facing board to provide a flush appearance. This is accomplished either by mounting (head and jambs) to a 1/2” channel stop, or nesting inside a 1/2” x 3/4” angle.
Unit of Measure