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)|
- Plate & shield
- Chain & shield
- Leather & shield
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).