Full-Backs: These are the highest quality boards found anywhere! At 42-Mil, these are twice as thick as regular backing boards! Made from 100% acid-free, virgin wood, cellular fiber, 42 mil. thick cardboard. Contains a 3% alkaline buffer and a pH content of 8.0-8.5. Sold in packs of 50. Measures 7" x 10 1/2". Mylites 2: The best raw plastic material available for permanent protection and storage! At 2 mil, these are twice as thick as ordinary Mylites! They are resistant to aging, decomposition, moisture, oxygen, and gas penetration; not affected by fungus, mold, or mildew; and immune to insect and rodent attack! Sold in packs of 50. Measures 7 1/4 x 10 1/2 plus 1-1/2" Fold-Over Flap.