Dimensions for mat board openings can be very confusing. It is very counter intuitive, but the opening of a mat board should never be the same size as the photograph or print being framed. Usually it is a little smaller and sometimes it is larger, but not the same. Most pre-cut mats are cut 1/2 inch smaller than the photo print. That allows the mat to overlap the image by 1/4 inch on both sides. That brings up another confusing issue. If you want 1/4 of overlap the mat window must be cut 1/2 inch smaller and not 1/4 inch smaller because there are two sides. Personally I think 1/4 inch is too much overlap and so do most custom framers so the default for prints that are smaller than 18x24 is 1/8 inch overlap on Frame Destination pre-cut mats. For example, an FDI mat for 11x14 print would be cut to 10-3/4” x 13-3/4”.
Sometimes people will do what is called float mounting where the mat opening is larger than the image being framed. This is done to ensure that no part of the image is covered up and also to allow for a signature below the image. When doing this it is common to leave a 1/4 to 1/2 inch gap between the image and the mat board for the top and sides of the print. That means that width of the mat opening will be 1/2 to 1 inch larger than the image. When leaving room for a signature the bottom gap will usually be 1/2 to 1 inch. That results in the height of the mat being 3/4 to 1-1/2 inches larger than the visible part of the image.
Frame Destination, Inc.