This article discusses the visual, but not decorative, aspects of mat design. There are many books written about this subject, so this is far from a complete treatise.
Generally, the total width of the mats (between the image edge and the frame sight edge) should be wider than the width of the frame. The purpose is to provide a visual break between the frame and the image so the image becomes the center of attention instead of having to compete visually with the frame.
Of course, this is not an air-tight rule as each image and frame combination presents a different design challenge. It depends, in large part, on the amount of “busyness” in the image and frame. If both the frame and the image are very busy looking, then the amount of visual space between them should be greater. If the image contains lots of negative space around the perimiter, then less of a visual break may be needed, even none. And, sometimes a very elaborate frame on a very ornate artwork works well without any visual break at all.
It reallly depends on one’s personal preferences while also considering the potential audience.
When mats are used, two mats are nearly always better than one. Three or more can be good or bad, depending on the colors and busyness of the arwork. If there are more than two mats, then the reveal (amount showing) of the lower mats should become narrower towards the artwork. For example if the reveal of a second mat is 1/4”, then the 3rd mat’s reveal should be less than 1/4” and so on.
I said this was not about decorative aspects, except for this one observation – stark white mats seldom do justice to the artwork. Mat colors should be chosen to complement the artwork – stark white nearly always distracts from the art.
-- Always do the Right Thing the Right Way the First Time - if you can figure out what that is! Ken, Spring Branch, TX