This is the classic mat that comes over the artwork.
The first step is to measure the amount of the artwork you want to show.
Be sure and measure in at least 1/8" from each edge (on larger pictures
we recommend a minimum 1/4"). This measurement becomes the inside dimension
of your mat.
Then determine the size of border you want on your mat. Generally, as the
size of the mat increases, the size of the mat border increases, too. We
seldom recommend mat borders less than 2". Sizes ranging from 16 x
20 and up generally get 2 1/2" – 5" borders. Traditionally, mat
borders are equal on the top and sides and 1/2" – 1" extra on
the bottom border. Visually this seems to make the mat borders look even.
However, many artists prefer to make all borders even so they can use and
reuse the mats on either horizontal or vertical pictures.