10 of 18x24 White Pre-cut Acid-free whitecore mat, fits 13x19 + back 10 of 18x24 White Pre-cut Acid-free whitecore mat, fits 13x19 + back This item comes with 10 single mats and 10 backing The opening size is 12.5"x18.5" for 13x19 print + backing board The mats are beveled cut with white core and 4-ply thick (about 1.4 mm or 1/16 inch).