What's a weighted mat? The window is moved up, so that the bottom weighting is heavier. The borders are 3.25" top and sides, with 4.25 on the bottom. These mats are vertical, so you can only use them with a vertical sized print.

Why weight the mat? Weighting mats helps to counteract an inherent human visual perception problem. If a picture is placed directly in the center of a mat that has equal width borders, the human eye will perceive that the picture sits oddly below center and it will therefore look top-heavy. A weighted mat compensates for this illusion and better balances the presentation.