What are the traditional rules for at photo mat measurements?

Question

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 × Y the mat should be some formula high, and some formula wide (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.

Answer 1

As I have learned it, the bottom border should be thicker than the other borders.

As with almost any rule of design or layout, this is not a strict rule to follow, just a guideline for what's balanced. If you want a different effect than a neutral, balanced frame, you can cut the mat any way you like.

Answer 2

Looking around, this is one of the more frequent methods I've found; same applies for both portrait and landscape: (Made a little guide as it is easier to read (picture from tumblr.com))

Giving us a result like this:

By calculating one could do – my geometry is a bit rusty so there is perhaps an easier way; but by taking advantage of angle by tan of opposite and adjacent side to the right triangle we get something like:

Having

a: mat width
b: mat height
c: image width
d: image height

We get

S: sides
T: top
B: bottom

By

S = (a - c) / 2

    a(b - d) + c(b - d)
T = -------------------
           4a

B = b - d - T

Alternatively B by:

     (3a - c)(b - d)
B = ------------------
            4a

---

Example

a = 21, b = 30, c = 17, d = 24

S = (21 - 17) / 2 = 2


b - d = 30 - 24 = 6

T = (21 * 6 + 17 * 6) / (4 * 21) = 2.71

B = 30 - 24 - 2.71 = 3.29