Beveled mats are a very decorative method of framing a picture.
   

To realize beveled mats, start cutting a Double mat the same as a single mat, making that first Window ("top mat") then cut the bottom mat" board 1 inch smaller than the "top" mat. You need to use the same outside edge to cut both openings. Use double-sided tape inside the window cut-out to secure the two boards together, in position.

Try to harmonize the Bevel of the mats with a particular tone in the picture.

Mats are to be judged by the cleanness of the cut and the even quality of the bevel. It is a skill which an unfortunately few number of artists or home framers possess. It is not difficult, but requires care and extreme cleanliness in the cutting of the mats and with a little practice, mats of professional quality can be produced.