Abstract: A window assembly comprises a prefabricated sheet metal main frame and a secondary frame suitable for use in supporting window units. A seat is formed in the main frame for seating the secondary frame in the window opening of the main frame. The seat has an entranceway opening laterally from a first side of the frame to permit lateral insertion or lateral removal of the secondary frame into and out of the seat through a first end of the window opening. A first stop is provided at the other side edge of the seat. The stop projects inwardly of the window opening and serves to prevent direct removal of the secondary frame from the seat through the other side of the window opening. A locking device is provided which fits within the other end of the window opening to cooperate with the main frame and the secondary frame to lock the secondary frame in the main frame when it is seated in the seat.