21x28 Matting for Picture Frame Black with Cream Core and 17x24 Opening Size (B221MAT) MAT SIZE: This matting fits a 21x28 frame. (frame not included) OPENING SIZE: This mat fits a 17x24 photo / print. (Actual opening size is 16.5x23.5) 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.)