Ready-made frames are available in standard sizes (page 7). All of the standard sizes are based on some practical purpose. For example, 5" x 7", 8" x 10", and 11" x 14", among others, are designed to coordinate with the photography industry. Some of the larger sizes are based on the size of a sheet of watercolor paper, or the most common sizes of stretched canvas.
Unfortunately, the art one chooses to frame doesn't always fit into these standard sizes. The fit may be comfortable vertically, but not horizontally, or perhaps the problem is the other way around.
Look at the drawings at the right, showing art placed in standard-sized frames with various possibilities for apportioning the mat borders. Personal preference is the most important guide for determining which is best.
Color also plays a role in the decision. If the proportions of the mat borders are not quite ideal, a mat color that blends with the art will make the size difference less obvious than a contrasting color.
The number of mats chosen for a piece of art is another decision influenced by the amount of space available. If the mat border will be very wide, using double or triple mats can help to break up the expanse. For a narrow mat border, just one mat is best, to avoid a distracting, striped effect.