Aspect ratio is mentioned for all rings. What is this, and what is it for?
The aspect ratio is how many times the wire diameter fits (in a line) over the inner diameter of the ring.
Aspect Ratio is often abbreviated to AR.
A ring with a wire diameter of 1 mm and an inner diameter of 5 mm has an aspect ratio of 5.
A ring with a wire diameter of 2.0 mm and an inner diameter of 8.2 mm has an aspect ratio of 4.1.
To write it scientifically: AR = ID/WD
AR = Aspect Ratio
ID = Inner Diameter
WD = Wire Diameter
What is this useful for?
This is useful for a few things.
Sizing up and down
Rings with the same AR behave the same in a chainmail pattern, even if the actual size of the rings is larger or smaller.
Suppose you have made a bracelet and you want to make it again in a smaller size. Wire is usually available in limited thicknesses, but how do you know which inner size works well? If the AR is the same, you know those rings work for that pattern, even if they are smaller rings.
If you cannot find exactly the same AR, a ring with a slightly larger size ratio usually works. A smaller size ratio may be too tight.
Communication
Another advantage is that many chainmail patterns online only list the AR that you can make them with, because it becomes too confusing to specify which precise sizes all work, especially when you deal with American sizes and different gauge systems.
Some patterns require a very specific AR. For JPL (Jens Pind Linkage), for example, the AR cannot be too small, or it will not fit; but if it is too large, the rings will not stay locked behind each other, and the chain will fall apart into Spiral.