Sometimes you need a mat. We carry ready-made mats in standard sizes and not-so-standard closeouts as well. The mat o.d. or outer dimension size is listed in the product number. For example: CM 11x14 shows mats to fit in an 11x14" frame. The pull down menu for "size" tells you what size the cut out window, or mat opening is. To determine what opening or window size you need, measure the paper size of the item to be framed subtract ½" from the each dimension, that's the size you need for an item so that it doesn't fall through the window. (An example of this is if you need a mat for an 8.5x 11" certificate, you need an 8 x 10.5 opening size.) Often we can create custom mats for you in volume about two weeks from when you place your order, call us for quotes.