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Getting Defensive: Armor Class

This article was first broadcast in Episode Twenty-Five on 23rd May 2018.

LennonAhh, there you are! What’re you doing down here in the basement?
Ryu:  Well, I’ve been flicking through Mordenkainen’s Tome of Foes, and I’ve gotta be honest I’m not sure I fancy our chances going up against those… things! Demons and devils, demogorgons and drow, it’s all gotten a little more dangerous.
LennonSo you’re hiding down here with a blanket?
RyuWhat? NO! I just–
OstronRyu?! Ryu?!
LennonI found her, she’s down here
OstronAhh, there you are! What’re you doing down here in the basement?
Ryu:  As I was just about to say to Lennon, I was just going through our armor and weapons and came across this old padding jerkin of my grandfather’s. He was an adventurer too, back in First Edition.
LennonOh it’s armor?! I thought it was a blanket!
OstronYeah, hard to believe that this would defend you against anything. I mean, that’s gotta be, AC 11? AC 12 at best…
Ryu: Well it is now, but that wasn’t always the case


See, there are many ways to defend against an attack in D&D. You could have a massive hit point pool, or maybe try to max out your saving throws, but the most basic defense of all is what what most people know as AC. Armor Class, as its name implies, measures the effectiveness of your armor, right?  Well, not exactly. In fifth edition especially, armor class represents your ability to ignore an attack, whether it’s because the attack missed, you evaded it, or it bounced off your armor. Damage mitigation by armor is completely ignored by the rules, which is why padded armor is so abysmal in D&D.  Historically this wasn’t always the case. The gambeson, or padded jack, was surprisingly effective armor. Its multiple layers of quilted linen greatly diminished the penetration of arrows and blades, turning deadly injuries into minor ones. The armor took considerable damage in the process mind you, but the wearer much less so.  However, since armor class only considers when the armor has completely prevented the attack from dealing damage, padded armor instead becomes the choice of the desperate, only barely better than being without armor at all. Any reduction in incoming damage is instead represented by a poor damage roll on the attacker’s part. Armor Class in 5e strictly represents the difficulty in landing an effective physical attack on the target.

So, with that being the case, why isn’t it just called “miss chance”?  Well, it hasn’t always been just a miss chance.  In first edition, there was significance to the type of armor you were wearing regardless of magic bonuses.  As with everything in 1e, there was a chart that granted bonuses or penalties to each weapon depending on the type of armor it was used against — a legacy of D&D’s tabletop war-gaming ancestry.  This chart helped to represent things like how strong chain mail was against swords, but also how weak chain mail was against damage from piercing or bludgeoning attacks such as spears and clubs. It also represent how certain weapons were tailored to breach particular types of armor.  As such, whether it was a +3 or entirely non-magical, the game was more concerned with what you were using, such as “leather armor with a shield”. The class of armor was just as significant as its final effective defensive ability.  Of course, this added yet another table to be looked up with every attack roll, but that’s the price you pay for that sort of realism, right?  And it was totally worth it, right? Welllll… Second edition kinda disagreed, and streamlined the table down to showing only how each armor class fared against slashing, piercing, or blunt (aka bludgeoning) damage, and those three base damage types can still be found in the game today. In third edition, this distinction was eliminated entirely, but new complexities were added.

In 3e, sometimes it was important to know when an attack actually landed or when it missed completely, regardless of whether it normally would have penetrated the armor.  Many “touch”-range spells ignored the armor of the target, which meant that the DM needed to know how much of a given target’s AC came from actual armor itself and how much came from evasiveness.  Another rule made it so that surprised characters couldn’t add their Dex modifier to their AC — they were too surprised to react, and so their only defense was the strength of their armor. These rules added two new stats to the character sheet: Touch AC and Flat-Footed AC. Touch AC was your Armor Class without any physical armor bonuses, and Flat-footed AC being your Armor Class without any evasion bonuses.  Together, AC, Touch AC, and Flat-Footed AC represented your total chance to be missed, your chance to evade attacks, and your chance to take a hit without being injured. While it added complexity, it didn’t particularly slow the game down since the numbers could be calculated beforehand. Fourth edition had other ideas, however.

Rather than having a single number for all physical attacks and using a completely separate mechanism for dealing with other forms of attacks, 4e used four numbers for defense, and like most of the rest of 4e did something that no other edition has done: it put AC and Saving Throws on the same footing.  When an attack was made, it would be compared to the target’s Armor Class as we’re used to now, but also to Reflex, Will, and Fortitude defenses. Attacks that could conceivably be blocked by armor would check AC, while attacks that needed to be evaded would be compared to the target’s Reflex defense. Fortitude defenses represented a character’s ability to resist physical attacks that armor didn’t help with (such as poison or knockdown), while Will was the defense of choice against mental attacks.  AC still held an element of evasion, at least for those wearing light or no armor, who could add the better of their Intelligence or Dexterity modifier to their Armor Class. By comparison, heavy armor wearers relied solely on their armor’s protection. However the distinction between Light+Dex/Int and Heavy was ignored when the time for the attack came, and no attacks targeting Armor Class made any distinction between whether contact was made or not. If evasion of the attack was important, such as avoiding a rust monster’s corrosive attack, it targeted Reflex instead. Then along came 5e.

5th edition took one look at 4th edition, and discarded all this baggage, in line with its aggressive streamlining and returning-to-its-roots mandate.   It reverted the non-armor defenses back to saving throws, and eliminated all distinctions about the type of hit made. Factors that would normally reduce a target’s AC instead grant advantage to the attacker, touch-ranged spells are re-balanced so the caster can hit without needing to calculate a separate touch AC for the target, and attacks that need to be evaded instead use a Dexterity saving throw.  As such, whether the force of an attack is deftly parried, nimbly dodged, blocked by a shield, or simply absorbed by the armor, it’s all simply a result of having an AC higher than the attack roll, and no game mechanics make a distinction about which happened. If a person’s defense lies in the inability of the attackers to land a hit that deals damage, they’ll likely have a high AC. This might take the form of the impenetrable armor of a knight, the unpredictable movement of a “drunken master” monk, the multiple flashing blades of the many-armed marilith intercepting incoming attacks, to even more exotic defenses. If an enemy has decoy body segment that can be struck without injury, making it hard to discern where to hit in order to deal damage, this too could be represented by a high armor class. Unlike every edition that’s come before it, 5e’s AC is, essentially, a pure value summarizing the character’s general defense, leaving DMs free to interpret if an attack roll of 10 vs an AC of 15 is a swing-and-a-miss, a deflection from the shield, a deft parry, or anything else you care to imagine.


Ostron: I see, that explains a lot… though I feel we’re missing out on other forms of defense — Saving Throws and Hit Points to mention just two
Ryu:  Yeah, we’ll get to those, there’s a lot of pieces to getting defensive — maybe we’ll cover those next week.

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