The opening size is 11.5"x17.5" for 12x18 print.

The mats are cutted at beveled.

The backing is also 4-ply.

The re-sealable adhesive strip of the clear bag is below the opening of the bag instead of on the flap.

 Our acid-free whitecore matboards are a 4-ply papermat that provide the aesthetic and design benefits of a bright white bevel., Each sheet of mat board has a richly colored front surface, a white core and a white backing. It can be used for matting photos, art, and needlepoint. Also great for crafts, backboards, signs, children's projects, and school projects.