16x20 matting. Opening is cut to 10.5"x16.5" to securely hold an 11"x17" photograph or art print in a 16x20 inch frame (frame not included). The matboard has a bright white core. Cut with a 45 degree bevel. 4 ply/.055" thick. Acid free and PH neutral. Colors are matched as close as possible but can vary depending on each person's screen settings. Matting gives your framed photos a professional look and also protects them from damage by keeping it from making contact with the glass where moisture would get trapped and warp/discolor your photo.