Delving into the oft-debated mechanics of Dungeons and Dragons (D&D), it’s hard to overlook the perennial discussion points: Armor Class (AC) and Hit Points (HP). While they may seem simple on the surface, these fundamental aspects of D&D often engender misunderstandings and heated debates. Armor Class, though instinctively viewed as a gauge of whether a character is struck or not, is in fact a more abstract concept. Following my recent post What are HPs in D&D anyway?, it’s fitting that we now unpack the intricacies of AC and its symbiotic relationship with HP - the two crucial pillars that underpin the game’s combat system.

Black and white art of two women dueling.
Rock, Paper, Greatsword!

The Genesis of Armor Class

The genesis of AC in D&D dates back to the game’s wargaming origins, where combat was typically a binary concept - either a unit was victorious or it was defeated. But as D&D emerged, focusing on individual combat encounters instead of unit battles, a more intricate system was called for. To build and sustain tension for individual characters throughout an adventure, a ‘tug of war’ mechanic was introduced, wherein the game sought to quantify both a character’s resilience HP and the time needed to overcome them. Hence, Armor Class was born.

In Original D&D (OD&D), AC represented the challenge of landing a damaging hit on your character. However, it left the concept of a ‘hit’ undefined. Was it a sword deflecting off plate armor? A deft evasion? A streak of divine luck? The rulebook intentionally remained silent on these specifics, emphasizing AC’s inherent abstraction.

It’s important to appreciate that the architects of D&D were well aware of the ‘swinginess’ of D20s. They understood that setting up a damage over time mechanic, regulated by the frequency of hits and the strength of attacks, was the most effective way to offset this inherent randomness. This allowed Adventures to be risky, but not random. An incredibly important element of TTRPG game design, if you wanted players to invest in their characters.

Armor Class: Defining its Abstraction

As the D&D editions evolved, the ambiguity of AC continued. Some players interpreted it as purely physical protection - the better the armor, the higher the AC. But armor could be bulky and restrictive. How would that account for the nimbleness of an unarmored monk? Or the ability of a rogue to evade attacks? Here’s where the abstraction comes into play.

AC is not just about physical armor. It encapsulates all the factors that might prevent a character from taking damage - natural agility, training, magical protection, divine favor, sheer luck, and yes, wearing a sturdy suit of armor. But, like HP, it’s not literally any of these things. It’s a mechanical tool that enables players to visualize their characters surviving in a dangerous world.

The Dance of AC and HP

The relationship between AC and HP forms the basis of the D&D Damage Over Time system. A character’s AC determines how likely they are to evade damage, while HP defines how much damage they can withstand before being defeated.

To illustrate, imagine a heavily armored knight (high AC, high HP) and a lightly armored rogue (lower AC, lower HP). The knight, with their superior armor, can absorb more hits before their HP depletes. In contrast, the rogue relies on their agility to avoid hits, mitigating the need for a large HP pool. Neither is invincible - the knight might face a foe with an accurate strike, and the rogue could encounter an enemy with an area attack that’s hard to dodge.

However, it’s vital to understand that AC isn’t a literal damage reduction. A higher AC doesn’t directly decrease the damage you take - but higher AC does mean you’ll take less damage over time. In this way, the single simple AC value serves the narrative role of damage reduction, as miss, or just toughing it out. Similarly, a high HP doesn’t mean you can shrug off a dragon’s fire breath - it represents your ability to avoid the worst of the attack, keep going despite the pain, or recover quickly.

Unshackling the Abstraction

It’s worth repeating that both AC and HP are mechanical tools for the game, not narrative shackles. They offer a game-friendly way to handle the complex, chaotic nature of combat without bogging down in ultra-realism that could hinder gameplay.

As the DM, it’s your job to help the mechanics flow into the narrative to best fit your campaign’s tone. You could describe the dodges and near misses for a rogue with high AC, or the punishing blows that a knight absorbs through heavy armor and sheer toughness. In other words, the mechanics tension and provides momentum to a scene without getting in the way of the narrative tone of the game.

Conclusion: Embrace the Abstract

Attempting to pin down AC in literal terms is a pure madness. Much like HPs, AC is best understood as an abstraction. Representative of various factors contributing to your character’s survivability in combat. It’s not simply the quality of a character’s armor, nor their ability to dodge, but a mix of physical, skillful, and even supernatural factors. It’s all of it, or none of it depending on how it fits the tone of your game.

Rules play a pivotal role in defining the tone, pace, and action in a TTRPG. At some point, most D&D fans question the abstract nature of AC and HP, wondering why armor doesn’t directly mitigate damage. While damage reduction systems can be enjoyable, D&D offers two broad alternatives: reduce damage per hit or leverage AC to apply damage less frequently. Both approaches yield similar outcomes, but AC achieves this without adding further mathematical steps, underlining its design simplicity.

And so, dear reader, as with Hit Points, I encourage you to embrace the abstract nature of Armor Class. It’s a tool, a game mechanic designed to simulate the unpredictability and risk of combat in a fantasy world. It’s there help build a narrative through game play, because you don’t need a lesson in impact physics, you need a trilling duel in the streets of Neverwinter! Use it as such, and weave it into your narrative in a way that enhances your game, not restricts it. Happy gaming!