There’s a little secret buried in the original AC system, one I’m not even sure Gygax and Arneson thought of. Gygax certainly seemed to have stepped away from the secret when he added negative ACs and a few other things. But it all stems from thinking about the AC numbers. Why does AC run from 2 to 9?
You can multiply AC by 10 to get a rough comparison of vulnerability.
A theoretical AC 10 would be 100% vulnerable. But 100% vulnerable sounds like “automatic hit” to me, and AC 10 definitely wouldn’t work that way in practice for the standard D&D combat system (chance to hit AC 9 is +1 compared to AC 8, +2 compared to AC 7, etc.) Same deal with AC 0, which would be “0% vulnerable” or completely impossible to hit.
It was this sort of thinking that lead me to abandon the modifiers “to hit” for things like cover or aimed blows and instead just divide the AC number by 2 or 3. For 50% cover, or aiming at someone’s arm or head, I’d use AC 5 or actual AC, whichever is better. For a smaller target, like the heart, I’d use AC 3. For a truly tiny target, it would be AC 1.
Now, I’ve been using a system in my monster stat blocks and dungeon modules for a while now where I abandon AC numbers completely and use the descriptors No Armor, Light Armor, Medium Armor, and Heavy Armor instead, so people who use either descending or ascending AC, or another system entirely, can all use my materials. But I still think of them as AC 9/7/5/3 behind the scenes, so I’ve been mulling over whether to do something about AC 1 for a while. And as I worked on some new material recently, I finally went for it, and created a new armor type: Extreme. There’s no wearable armor equivalent. It’s reserved for enchanted, magical, or demonic creatures.
There’s one other armor type beyond that… not AC 0, technically, for the conceptual reasons I previously explained. It’s the armor type I’m calling Abstract, because it’s used for things like “attacking someone’s will to live”. I’ll be using it eventually when I tackle psychic combat. Base chance to hit is 1 in 20.
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