Armor Class: The Good, The Bad, and The Ugly

Armor Class is one of those D&Disms about which there are three reactions:

1. Grudging tolerance.

2. Unadulterated hatred.

Well, that's all three. Myself, I'm of two minds on it. First and foremost, like many "D&Disms," Armor Class is an abstraction that "just works." It does what it's supposed to: make fighters and clerics more survivable.

Second and less foremost, Armor Class does this in a silly way that many, many people find dissatisfying as a gameplay mechanism, myself included.

The Good

In almost every traditional RPG, systems for injury and armor are a core part of design. (This is because for the greater part of RPG history, traditional RPGs have been combat engines with a few nods toward roleplaying tacked on.) The systems for injury and armor tie into one another because you want rules for what happens when a character catches a stray arrow and rules for how armor keeps him from bleeding out after such an occurrence.

Hit Points answer the question of how you decide what happens when an arrow worms its way between your ribs, and Armor Class answers the question of how armor keeps that from happening. It's a simple, elegant solution: armor makes it harder for that arrow to find its mark. It's an accuracy debuff.

By keeping the system simple, D&D allows players and GMs to field a number of characters on a single battlefield without much issue. For each attack, a character rolls 1d20 and compares it to a single value to determine a hit or a miss. Fast, easy, no fuss. Magical weapons make it more likely for a character to hit, and magical armor makes it less likely that a character will be hit.

The Bad

That's the good stuff. The bad stuff is that there's no guarantee of this being the case. Armor Class relies on a system of "effective HP" where characters are probabilistically more resilient because attacks are likely to miss.

If Orc A has +0 to-hit and does 10 damage on a hit, and Fighter B has 11 AC and 10 HP, how many attacks can Fighter B survive? It might be 0, it might be infinity! Orc A has a 50% chance to hit and kill Fighter B. If Orc A has an unlucky streak, Fighter B can survive indefinitely since each attack is all-or-nothing. If Orc A gets a single good hit in, Fighter B is toast, and his AC has meant nothing. We can tweak these numbers to give Fighter B a better chance of survival; say his AC becomes 20, so Orc A has but a 5% chance of striking the killing blow. Better, right? Yeah, unless Orc A gets a lucky hit in, in which case Fighter B's armor has done him no good at all. (One could argue that the "lucky hit" issue is prevalent in any system; I rebutt that it is exacerbated in D&D.)

Compare this to a system in which armor reduces damage. Let's change the scenario slightly: Orc A has a 100% chance to hit and cause 10 damage on Fighter B. But here's the rub! Fighter B now has armor that reduces Orc A's damage by 1. This means that Fighter B has a 100% chance of taking damage from Orc A, but he has a 100% chance of surviving at least one hit. Meaning that even with the worst luck in the world (fighting a probability-bending orc whose damage equals his maximum HP), Fighter B's armor still keeps him alive.

This also runs straight into the issue where Armor Class barely (or simply doesn't) keeps up with monster attack bonuses, placing "tanky" characters like fighters into a situation where they are hit with great frequency despite their superior AC. With D&D's relatively static combat system, it is not particularly enjoyable to be the fighter on the frontline watching your HP tick down as you pray your turn comes around before you hit 0 HP.

The Ugly

Perhaps the worst part about Armor Class is how it feels at the table. Set aside for a moment all arguments about what a "realistic" (verisimilitudinous) armor system is. Instead, for many players (myself included), Armor Class doesn't feel right. The mechanic does not act in the manner that approximates our conception of armor, which drives us to develop or pursue alternatives.

Armor Class does not distinguish between "hard to hit" (evasion) and "hard to hurt" (toughness), and it is mildly infuriating. Simply put, I am a stubborn ass when it comes to RPGs. I do not need mechanics to be one-to-one approximations for real life, but if you're using a system where the majority of mechanics center around combat, I think making a slight distinction between dodge tanks and armor tanks is valuable. The fact that in D&D 5e a rogue and a cleric have similar Armor Classes drives me up the wall because they ought to be opposites: rogues ought be hard to hit and easy to hurt, whereas clerics should be easy to hit and hard to hurt.

The Solution

There isn't one because, like I said, Armor Class "just works." I'll discuss some alternatives in the next blogpost.