THAC2, OD&D AC, and Combat Ranks

In OD&D, there are only 8 armor classes. AC 9 is the worst (unarmored) and AC 2 is the best (plate & shield). Each number maps reliably back to a given armor type. So, presented with AC 5, you know that means chain armor (or medium armor, if you’re abstracting it).

Probably medium armor, depending on your setting (source)
  1. Plate & shield
  2. Plate
  3. Chain & shield
  4. Chain
  5. Leather & shield
  6. Leather
  7. Shield
  8. Unarmored

There are no other armor classes in the whole world. If you roll well enough on your starting gold, you can begin at first level with the best armor class in the game (plate armor and a shield cost 60 GP, well below the expected value of 3d6 * 10 starting GP, which is 105 GP). Contrast this with the cost of full plate in Second Edition: 4000 – 10000 GP.

Magic armor does not modify AC, but rather penalizes the attack roll, and the most potent magic armor in Men & Magic is rated +2. By the book, magic shields only help one third of the time, and only if the magical bonus of the shield is greater than that of the armor (i.e., they don’t stack). That requires an extra die roll per combat (extra fiddly), and so will almost certainly be something I jettison. Maybe I’ll house rule magic shields to an additional flat penalty of 1 to the attacker’s roll, along with the full magic bonus for certain saving throws (like dragon breath). I’ve always liked the idea if shields being extra good against dragon breath. [Edit: see here for a clever way from Talysman to handle the shield chance without resorting to another die roll.]

In addition to this form of fixed descending AC, OD&D uses something that I referred to in a previous post as a matrix of combat ranks (my words, not from the book) rather than THAC0 or attack bonus. All classes move through the same ranks, but fighters move through them faster (advancing to the second rank at level 4, when attaining the “hero” title). From Men & Magic, page 19:

Magic-users advance in steps based on five levels/group (1-5, 6-10, etc.), and Clerics in steps based on four levels/group (1-4, 5-8, etc.). Normal men equal 1st level fighters.

I actually like this staggered progression (which is preserved in Moldvay Basic), because it means that PCs need to survive by their wits for a while before they get any mechanical advancement (though I can see why some people might like a smoother progression, and in fact using Target 20 with a smoothly advancing attack bonus seems to be one of the more common OD&D house rules).

Attack Matrix 1

The OD&D approach does have some drawbacks compared to both THAC0 and armor class as target number (more commonly known as ascending armor class). THAC0 is easier to reason with than attack matrices, and direct target numbers don’t require any math (other than situational bonuses and penalties, though in practice those modifiers can end up being rather complicated in 3E).

In the end, all these systems are about the same level of complexity, and all require writing the same amount of numbers on the character sheet. In OD&D, you write down your attack rank column (which is a list of target numbers). In 2E, you write down your THAC0 (and probably derive your other target numbers from that). In 3E, you write your base attack bonus adjusted by all the other modifiers next to every weapon (if you are efficient).

However, the matrix approach does have some benefits, the main one being that anchoring AC helps prevent absurd bonus inflation (especially coupled with the simplicity of ability scores in OD&D). This helps make it clear that while mechanical combat advancement is part of the game, it is not the biggest part of the reward structure. Also, simple AC categories may help make weapon versus AC possible (though you do have to deal with rulings about monster hide being similar to what kind of armor).

This is one reason why I don’t care for the extra d20 SRD armor types that managed to creep into Labyrinth Lord. They break the elegant simplicity of the light, medium, or heavy armor types (modified by a potential shield) present in OD&D and Basic D&D. Obviously there are more types of armor than leather, chain, and plate; however, it does not seem useful to make a distinction regarding armor class past that threefold categorization (other differences can be handled by ruling; for example, chain mail could be used as a crude filter, something that it would be difficult to do with hide armor).

7 thoughts on “THAC2, OD&D AC, and Combat Ranks

  1. SAROE

    Should shields be considered part of the Armor Class? Or as a penalty to the attack roll? Especially if you are going to use Weapon vs. Armor Type. Or you could do what they did in Dragon Warriors and let a shield user roll a D6, on a 6 (or maybe it was 1), the shield takes the blow. Perhaps higher level fighters could improve on that number as a class feature.

    1. Brendan

      A bigger penalty to the attack roll, perhaps to one enemy only, would be a totally reasonable and interesting way to handle shields, in my opinion. This would model the fact that a shield is really quite useful against a single opponent.

      Doing that would make combat a bit more complex though.

    2. Gibbering Mouther

      I can’t help but think that if one were to really “model” shields one could not overlook that they’re viscous weapons. Heck the Roman shield was basically a way to disarm and/or mangle an opponent at close range. I’d think a trained shield user would get and AC bonus (vs. missiles)/big enemy hit penalty (in melee) and an extra attack every other round?

      Yet fun comes first, and the current shield rules provide a good bonus that is easy and logical. Plus the giving up a two handed weapon in exchange for better protection is a reasonable gamble.

    3. Brendan

      I would not give an extra attack (remember that OD&D combat is supposed to be pretty abstract); I don’t even give extra attack rolls for wielding two weapons (just a +1 to hit).

      However, a shield could certainly be used as a weapon, even if it was the only thing carried. I think the absolute effectiveness would be situational, as a shield in most cases is probably not as good as other weapons (Captain America notwithstanding).

  2. Aplus

    The elegance was almost immediately broken when AC bonus from DEX was introduced in Greyhawk. But I agree, I personally like the way the LBBs do it, and it makes switching between RPG and wargame very easy.

    I did make up a slightly modified man-to-man combat table and use it in a game once, but the players said they preferred the d20 to-hit roll.

    I like that shield blocking rule mentioned above as well. One might also grant a chance to dodge for high dex rather than screwing with the LBBs’ beautiful AC system…

  3. The Recursion King

    I play the D&D Cyclopedia every two weeks and in my campaign I use only 3 armour types: Camelleathers (AC 7), Snakeskin (AC 6) and Beetleplate (AC 5). My campaign is in a desert setting where metal armour is not much use anyway. Beetleplate is rare: someone must slay a giant beetle to get the shell to make it.

    I use shields in the game, making the best AC 4, plus 3 points of Dexterity bonus, AC 1. I use the house rule of shields can be shattered to absorb damage, making the shield extremely useful, especially as the AC cannot be upgraded past 1. And by cannot, I mean, there are no magical armours, no rings of protection, nothing, that can push an AC 1 to better than 1 in the campaign.

    This allows me to throw in one of Dave Arneson’s house rules: AC negative (e.g. -1) needs a positive magical weapon to hit (e.g. AC -1 creatures are immune to any weapon that is inferior to a +1 weapon). I’ve expanded this so that AC 0 is special and can be hit by silver or magic weapons only and revised the AC’s of enemies to follow suit: werewolves are now AC 0. Also, I rule that anyone who wants to strike AC -1 with a magic weapon must roll against AC 0 to hit, as this becomes the hardest roll to make, a kind of ‘max AC’.

    Add all this together and it’s working a treat. A nice, distinctive campaign where AC inflation is non existant.

    1. Brendan

      That sounds like a great campaign. I especially like the labor required to obtain beetleplate.

      The use of ACs 0 and -1 to indicated special defences is also interesting, and not something I have seen before. I might adopt that myself, since it also allows one to keep monster stat blocks shorter.


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