Pack of 50 Black 16x20 Mats for 12x18 Photo + Backing + Bags Pack of 50 Black 16x20 Mats for 12x18 Photo + Backing + Bags This item comes with 50 mats, 50 backing, and 50 bags The opening size is 11.5"x17.5" for 12x18 print. The mats are beveled cut with white core and 4-ply thick (about 1.4 mm or 1/16 inch).