Our acid-free White matboards are 4-ply and feature a white core. With an overall size of 26x26, these mats with Square openings fit perfectly into 26x26 frames and are bevel cut to showcase your 21x21 photos. Actual mat opening size is 20.5x20.5 inches so that it may safely and securely house your 21x21 photo, artwork, needlepoint, sign, etc. This pack includes 1 Single Matboard Show Kit With Acid Free Backing & Clear Bag. Proudly made in the USA by Country Art House!