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