10 of 22x28 White Pre-cut Acid-free BLACKCORE mat, fits 17x22 + back 10 of 22x28 White Pre-cut Acid-free BLACKCORE mat, fits 17x22 + back This item comes with 10 single mats and 10 backing The opening size is 16.5"x21.5" for 17x22 print + backing board The mats are beveled cut with black core and 4-ply thick (about 1.4 mm or 1/16 inch).