How to order or size your matting
The mat sidings depend on your preference and is usually even for all sides. In the illustration, I made it 2 inches on all sides. When you order your mat and backing, you can go narrower or wider. Specify your desired external dimension and the size of your mat window or cutout.
For example:
External dimension is 12.75 x 15.75 inches.
Mat Window is 8.75 x 11.75 inches
The craftsman will know to center your opening and make the siding widths even.
The base or mounting board and the backboard or back panel should have the same external dimensions as that of the matting.
If you are making the mat from scratch, you can buy matting boards from school supplies. I would suggest you do not use illustration board as the black part has a tendency to come off and stain surfaces.