Our Acid-Free matboards are 4-ply and feature a White Core. With an overall size of 18" x 24", these mats fit perfectly into 18" x 24" frames. Each mat is bevel cut to enhance your 12" x 18" photos. The actual mat opening size is 11.5" x 17.5" inches so that it may safely and securely display your 12" x 18" photo, artwork, needlepoint, sign, etc.