Armor Class Types Cheat Sheet
A taxonomy of how reality says “no”.
AC
This represents your defense against attacks from monsters.
|AC|
This represents an AC that can only be beat by rolling a value equal to the AC on a natural die roll. Luck effects, ++’s, and other die setting effects can modify this. For |AC|’s over 20, you must be able to hit the group with the attack in order to hit the attacked target.
iAC / iiAC / iiiAC / iⁿAC / etc.
This represents an infinite amount of normal AC. Rolling a 20 on your d20 to hit, sources of iTH, etc. will hit this AC. Every |AC| 20 equals iAC 1. This is similar for ||AC|| and iiAC, |||AC||| and iiiAC, etc.
||AC||
This is similar to |AC| except at ||AC|| of 2, you must be able to hit the legion with the attack in order to hit the attacked target. With ||AC||, if a target is casting a spell against you, they must roll to hit you to affect you. This AC ignores all die setting effects, but luck effects and ++’s still work. Every ||AC|| 20 equals iiAC 1.
|iAC|
This represents an infinite amount of |AC|. This means you need a source of iTH and must roll a natural 20 on your d20 to hit.
|||AC|||
This is similar to ||AC|| except that your d20 to hit must use nothing else. No luck effects, ++’s, or other die setting effects will work.
|…^∞|AC|…^∞|
This represents an infinite amount of |’s of AC. This usually means you cannot be hit by attacks, spells, psi, or innates (group or otherwise) unless the attacker has some way of ignoring or converting this AC type.
iiAC
This represents an imaginary number of iAC. Normally not seen outside of Ultraplanar beings, this AC type requires an irrational or imaginary amount of iTH to beat.
Stun AC
This represents the number of digits of normal TH you must have to beat this AC.
(5e) AC / (5e) iAC / (5e) |AC| / etc.
This only cares about the attacker’s last two digits of their TH. Similar logic applies for (5e) iAC, (5e) |AC|, and related types.