15x21 Super White Custom Mat for Picture Frame with 11x17 opening size Mat is standard 4 ply material used in Custom Frame shops. Matting material is Bainbridge mat board. Made of lignin-free and pH-neutral materials. Fits into a(n) 15x21 Frame. Frame options can be seen in the accessories listed below. This mat will fit a 11x17 Photo. Actual opening size will be 10.75x16.75. NOTE: OPENING SIZE(s) MAY BE CHANGED BY CUSTOMER. Please include your desired opening size(s) by email, or through the contact seller link. If we do not receive a request for a custom size within 2 hours of your order it will be cut with the opening size listed above. Feel free to contact us with any questions you may have.