A one-piece metal frame fin is locked to the wooden frame members of a window unit by exterior cladding. A notch in the frame members receives the inner portion of the nailing fin while the outer portion of the fin is recessed into the wood when the fin is in its closed position for storage, transport and use in abutting frame members of multiple window units. An L-shaped corner nailing fin is carried on one of the frame fins and can be pivoted into locking engagement with the adjacent frame fin during installation of the window unit. In the case of joined multiple window units a mullion cover extends over the adjacent ends of the frame nailing fins and is locked to the outer nose portion of the exterior cladding. Use of foam sealing strips eliminates the need for on the job site caulking.