I learned about this in primary school; but that is some years ago so I do not recall exactly how it was. I understand there is no correct way to do this – but anyhow we learned the "correct way". :)
I remember something in the direction of:
Having a picture of
X × Ythe mat should be
some formulahigh, and
some formulawide (if creating frame as well as mat). We also learned, as I recall, that the mat opening should be a tiny bit above, or below, centre; one border should be slightly thicker. This to drag the eyes downward, or to weight the picture down.
What is the standard way to measure this? I understand there is no correct way to do this — but as we learned some concrete numbers to work with — and they gave good results and would think there is some basic guidelines for this.
My most important question is which border should be thicker – and by how much.
I got really confused finding this post on eHow (see step 5), where they state the width should be greater. I definitely remember it was either top or bottom.