10 of 18x24 White Pre-cut Acid-free whitecore mat for 12x18 + back+bag 10 of 18x24 White Pre-cut Acid-free whitecore mat 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 mats are beveled cut with white core and 4-ply thick (about 1.4 mm or 1/16 inch).