A historical look at D&D ACs (part 1)

[MerricB]

Historically, Armour Class is derived from naval games (Fletcher Pratt, Jane's Fighting Ships) which Dave Arneson incorporated into his own naval design, and then passed on to D&D.

Original D&D Armour Class is not a feature of Gygax's and Perrin's Chainmail. Instead you have a table that cross-references the type of armour with the weapon used, the combinations are as follows:

Armours: None, Leather/Padded, Shield, Leather + Shield, Chain/Banded/Studded/Splint, Chain+Shield, Plate, Plate + Shield

By Original D&D, you get AC of 2-9 and the "to hit" tables give only those values. Of note, Dexterity gives *no* modifier to AC, a magic suit of armour subtracts its value from the "hit dice of the opponent", and magical shields have only a 33% chance of working (in which case they give their penalty to the attacker).

Supplement I: Greyhawk has a much greater range of magical armours, and has one of the clumsiest explanations ever as to how they work, using this table:

(It also has errata: Chainmail and +1 shield should be AC 3)

It's in AD&D that we finally get the cleaned up tables, AC now goes from 10 to -10 (rather than 9 to 2), and it'd stay that way (mostly) for the next twenty years, or thereabouts.

One of the interesting features of this is that every AC in original D&D (sans supplements) is in the range of 2-8.

With the expanded AC ranges from Greyhawk, you get the Will-o'-the-Wisp, with an AC of -8, the highest on the supplement's table, but there aren't many other monsters with negative ACs (the Platinum Dragon, with -3, is the next best).

Blackmoor is back in the 2-8 range, Eldritch Wizardry gives the first AC of actually 9 (the Succubus), and gives Demogorgon a fearsome -8 AC, with Orcus behind on -6. Finally, Gods, Demigods and Heroes incredibly goes back to the 2-9 range, with a few exceptions (and not gods - generally monsters), though some of the gods have magic armour that isn't factored into their AC - Odin has a helm +5 and mail +5!

So, onto AD&D where Gygax is drawing on the various work he and others have done for D&D. There's no AC 10 in the Monster Manual (but, to be fair, there's no AC 9 in the monsters in core OD&D either!) The Will-o'-the-Wisp retains its AC of -8, likewise the Platinum retains -3. Demogorgon and Orcus keep their respective ACs.

The tables in the DMG enshrined the -10 to 10 range of ACs, and a monster came out with a -10 AC not all that long after - Dave Sutherland's take on "Lolth" - from Q1. Also in 1980, RJK's and Jim Ward's revision of the deity supplement gave the gods ACs that weren't generally quite as impossible - Odin has AC -6 instead of his previous (effective) -8, although we get our first breaking of the -10 "cap" - Indra has an AC of -12!!

Indra's AC is actually correctly calculated for his +4 plate, +4 shield and 25 Dex (-6 def bonus).

However, it's debatable how many of these really low ACs belong to beings that will be fought...

(more soon)

 

[MortalPlague]

interesting read! Thanks for compiling all the information, Merric.

 

[billd91]

However, it's debatable how many of these really low ACs belong to beings that will be fought...

Lolth is pretty much the only one that I can recall any serious expectation of getting into a fight with PCs. And her extreme AC is balanced out with low hit points (66 if memory serves). Tough defense, relatively glass jaw.

 

[Tequila Sunrise]

...the Platinum Dragon...

With all the Monstrous Manuals and Draconomicons that we have, why have I never seen this? We have dragons made of manmade composite metals, dragons made of magma, Chinese dragons, dragon-people, dragon-things, and even cute little purple dragons...but the platinum hasn't been written down for potential killing since OD&D?

What's up with that?!

 

[MerricB]

With all the Monstrous Manuals and Draconomicons that we have, why have I never seen this? We have dragons made of manmade composite metals, dragons made of magma, Chinese dragons, dragon-people, dragon-things, and even cute little purple dragons...but the platinum hasn't been written down for potential killing since OD&D?

What's up with that?!

The Platinum dragon is Bahamut. He was certainly available for killing in AD&D. I'm not so sure if he gone Monster Manual stats or only Legends & Lore stats in 2E.

Cheers!

 

[MerricB]

Lolth is pretty much the only one that I can recall any serious expectation of getting into a fight with PCs. And her extreme AC is balanced out with low hit points (66 if memory serves). Tough defense, relatively glass jaw.

My favourite story of Lolth is of a thief who backstabbed her when she'd already cast fire shield. The thief slew Lolth - dealing x5 damage or thereabouts helps - but the thief died himself, as he took *double* what Lolth took. Ah, the old Fire Shield spell!

Cheers!

 

[Olgar Shiverstone]

You've clearly missed the Kevlar Dragon ...

 

[MerricB]

One of the very interesting features of having almost every (fightable) monster in oD&D/AD&D being in the 2-9 range of AC is that it makes all characters relevant in physical combat, something that quickly became not the case when you dropped into the negatives.

Speaking in terms of THAC0, first level magic-users and thieves in AD&D had a THAC0 of 21; this translates into a THAC2 of 19 and a THAC5 of 16. For the thief, this doesn't change until the character reaches 5th level, at which point it improves by a massive 2 points (THAC2=17, THAC5=14). At 9th level, the thief's next point of improvement, this improves by 3 (THAC2=14, THAC5=11).

Meanwhile, the fighter has been improving by 1/level from a start of THAC0=20, so by 9th level, he's at THAC2=9, THAC5=6.

I look at AC5 because it's a pretty standard AC for low-level monsters in the game (and some higher-level ones as well), whilst AC2 is about as high as your regular foes will get.

Once you move outside those values - above level 9, and below AC 2 - the mathematics of AD&D combat begins to look a lot more shaky.

More soon...

 

[howandwhy99]

As I understand it AC is a measure of hardness for solid matter. As a piecemeal variation on cover it affects "to hit" odds, but it doesn't need to be the only factor upon a 'to hit' roll. (For example, AC doesn't alter the target number's odds against gases or liquids in quite the same way, sort of like touch AC and bee attacks)

Dexterity could affect the target number (which isn't the same thing as the AC).

Magical armor modifiers can affect an opponent's 'to hit' roll. The opponent's magical weapon modifiers thus affecting the target number or possibly the AC, I think that's largely a house rule on how magic works by campaign.

Armor proficiency affects the target number too in regards to the user knowing how to make the armor work in his or her favor when defending.

AC can represent hardness of skin, leather, wood, ceramics, stone, metal or other solids. Woven substances, like chain or splint, change the configuration of a suit of armor altering its easiness to learn, its pliability for movement, durability, and AC rating for example.

Also a lot of variation exists within single types of materials, like the hardness of lead compared to steel even though both are metals and collapsed within a single AC number for ease of use.

Because the numbers represent a hardness scale negative and zero AC didn't make sense and weren't used Chainmail IIRC. Negatives could still be understood as magical modifiers upon an attacker's roll, but an attacker could never "hit zero" just as hardness could never reach an infinite state either. (Think of how a natural 20 didn't always equal a hit) The standard 2-9 range covers the -8 maximum.

Attack type (usually meaning mundane weapon Bludgeoning/Piercing/Slashing) vs. AC type is based upon the AC's particular substance and its ability to protect from another substance's collision. The shapes of both were factored in. So we have the shape of the weapon like a dagger point affecting the shape of an armor like chainmail differently than that of plate mail even though all three are probably entirely steel.

So you can see, a whole bunch of factors can be included into a rating with the numbers still being relatively small and easy to remember. It's when the unusual cases occur that the DM needs to remember when to modify the roll or target number ultimately. But I find those little nuances can really make the game too.

 

[AbdulAlhazred]

One of the very interesting features of having almost every (fightable) monster in oD&D/AD&D being in the 2-9 range of AC is that it makes all characters relevant in physical combat, something that quickly became not the case when you dropped into the negatives.

Speaking in terms of THAC0, first level magic-users and thieves in AD&D had a THAC0 of 21; this translates into a THAC2 of 19 and a THAC5 of 16. For the thief, this doesn't change until the character reaches 5th level, at which point it improves by a massive 2 points (THAC2=17, THAC5=14). At 9th level, the thief's next point of improvement, this improves by 3 (THAC2=14, THAC5=11).

Meanwhile, the fighter has been improving by 1/level from a start of THAC0=20, so by 9th level, he's at THAC2=9, THAC5=6.

I look at AC5 because it's a pretty standard AC for low-level monsters in the game (and some higher-level ones as well), whilst AC2 is about as high as your regular foes will get.

Once you move outside those values - above level 9, and below AC 2 - the mathematics of AD&D combat begins to look a lot more shaky.

More soon...

Right, but OTOH those super low ACs weren't likely to factor in a whole lot. Most creatures in that category weren't going to be defeated by the fighters hacking at them anyway. They were going to go down to magic, and probably not even by direct magical attack, but by 'indirect' means (IE like dropping a wall on them or collapsing the ceiling of a cave, or sticking them fast with rock to mud, etc). Heck, things like demon lords all have practically unlimited teleportation and/or gating capability. At best your post level 12 fighters might manage to be in for the final kill once the monster was somehow disabled in some way.

So, yes, melee combat in a sense 'breaks down', but it hardly matters. No sane wizard would stick around to play pokey with a monster in the deep sub zero AC category if all they were going to be doing was sticking it with their dagger. That would be true even if the monster's AC was 2 in all likelihood.