16x21 Matting for Picture Frame Black with Cream Core and 12x17 Opening Size (B221MAT) MAT SIZE: This matting fits a 16x21 frame. (frame not included) OPENING SIZE: This mat fits a 12x17 photo / print. (Actual opening size is 11.5x16.5) Matting Material is Crescent Mat Board. Black Finish / Cream Core with White Backing (PH Neutral). This listing includes (1) Custom Cut Mat. (The size selected represents the OUTSIDE dimensions of the matting.)