Armor Class AC is a rating of how much armor your character is wearing.
This number ranges from 100 to -100, with 100 being the worst and -100 being
the best. The lower your AC the harder it is for others to hit you with
a hand-to-hand attack. What this means is that AC has no bearing on spell
casting. While it is possible to get your AC lower than -100, this has
no bearing on the game in any way.
The armor class scale is:
100 Naked person
0 Heavily armored
-100 Divine armor
Equipment can affect your AC in two ways: an Armor affect or an AC-apply
affect. The number in the Armor affect is a direct change to your AC Any
piece of equipment can affect your AC in this way. The number in the
AC-apply affect, which appears only on equipment of type ARMOR reduces
your AC more if the equipment is worn on the body than if it is worn
Example object identification:
AC-apply of 5
-10 to ARMOR
This piece of armor will reduce your AC by 10 no matter where it is worn;
it also has an AC-apply of 5, which will reduce your AC by a multiple of 5
depending on which equipment slot it takes up. Note that for the ARMOR
affect, negative is good (reduces your AC For AC-apply, positive is good.