Our acid-free Textured Black matboards are 4-ply and feature a white core. With an overall size of 21x21, these mats with Square openings fit perfectly into 21x21 frames and are bevel cut to showcase your 17x17 photos. Actual mat opening size is 16.5x16.5 inches so that it may safely and securely house your 17x17 photo, artwork, needlepoint, sign, etc. This pack includes 1 Single Matboard. Proudly made in the USA by Country Art House!