A mat is the border around the painting. It allows air circulation between the painting and the glass so that the painting will not be damaged by moisture. Mats are cut from 4-ply or 8-ply matboard, and more than one mat layer may be used. Often each layer is a different color in order to line the picture with a specific color in the painting. The mat color may be based on the colors in the artwork, the surrounding wall colors, or just personal choice. Mats are cut in varied proportions that allow the painting to make the artist's statement. For example, a mat might be used to increase the size of a picture to make a more dramatic display. Mat width should be different than the picture frame width. Mats are available in standard sizes or they can be custom cut.