Armor Class

Armor Class (usually abbreviated AC) is a demented RPG measure of how tough you are to hit. Not only is it a half-baked idea, but it's rather perverse -- low is good, high is bad. Armor Class is the "ac" in thac0 (to hit armor class zero).

A naked human is usually said to be AC 10, with clothing you get AC 9. Leather armor gives around AC 7, plate mail 3, and dragon scale mail easily gives -4 or better, depending on the age of the dragon it was taken from. The armor class system is also used in NetHack.

Shadowrun and the other games like it have a much better means of describing how well-protected a being (or structure) is from attacks and incidental damage; in fact, I once tried to transplant the entire Shadowrun combat system into one of my AD&D campaigns (there's a lot to it, I'll just say that). This hybrid system is also an integral part of the MUD I've been coding for the last five years (it WILL not die!).

Two things should be said about Armor Class. First, as of 3rd Edition, it goes up now (thank you!). Second, though the language used confuses this, it's been officially said that AC isn't meant to show how hard you are to hit; it's supposed to show how hard it is to land a hit that actually hurts you. Sure, you're still technically hit if someone clubs you in the back while you're wearing full plate, but...

I will grant that AC isn't the most realistic mechanic in the world. However, it is fast and simple, so if I were running a D&D game, that isn't one of the rules I'd try to rework.