If It's Worth Remembering....
It's Worth Framing!
Double Mat
Double mats allow you to add an accent color that can be used to help draw attention to the art by outlining it. However, a double mat can be made using two of the same color for a more subtle look.
Single Mat
When mats were first introduced to framing, they were all a single layer. Today it is much more common to use two or three layers. Single mats are probably used most on high end art where the frame designs are simple and classic, not decorative. Single mats also have application when matting vintage, classic or antique pieces as it helps them look authentic to their era.
When mats were first invented, they had two core functions. One was to provide an area for visual relief so the art could be viewed without the distraction of the nearby surroundings. Secondly, the depth of the mat also serves as a spacer to keep the glass from touching the face of the art.

In the past few decades mats have taken on a more decorative role. Since mats are now available in many colors, patterns and textures, they are used to match colors in the art and to coordinate with the room decor.

Mat borders are also an important consideration. It is best to avoid having mat borders the same width as the frame surrounding them. Generally it will look best if the mat is somewhat wider than the frame. Too narrow a border can bring down the perceived value of the art and it tends to look crowded in the frame.

Most mats have square or rectangular window openings, but other cuts are possible. The opening can conform to irregular art shapes. An angled corner (octagon) can cover damaged corners on the art, etc.