BLACKCORE MAT, 10 of 18x24 White Pre-cut Acid-free for 12x18 +back+bag BLACKCORE MAT, 10 of 18x24 White Pre-cut Acid-free for 12x18 +back+bag This item comes with 10 single mats, 10 backing, and 10 bags. The opening size is 11.5"x17.5" for 12x18 print + backing board + clear bag The white mats are beveled cut with black core and 4-ply thick (about 1.4 mm or 1/16 inch)