Flush bolts can be used on doors that need to be secured without having a mullion or wall for latching. Flush bolts are usually used on a pair of double doors, like a large closet in a commercial building or French doors. These doors may have a lock, but the flush bolts allow one door to be anchored to the top of the door and the floor. The other door can then latch to the anchored door and keep the opening secure when not in use. Flush bolts are recessed into the top and bottom of the door edge and slide upward or downward into the jamb or threshold to keep the door secure and closed. Fully automatic, inactive door is latched, bolts are extended when active door closes, door is unlatched, bolts retract when active door is opened. Low actuation forces, top bolt has no spring tension. Fits standard ANSI A115.4 door and frame preparations. Bolt throw is 3/4" with a 7/8" vertical adjustment. UL Listed 3 Hour fire doors 8'0" X 8'0".