Armour Class combines a few different things in D&D: dodging, parrying, and armour. I could be AC 5 because I have a high Dexterity (dodging), or I could be AC 5 because I'm wearing chain mail (armour). I don't recall the specifics, but I could also lower my AC in some editions by spending the round only defending (parrying).
One thing which can't make your AC better is skill. Wait, what? That's right - in D&D, it is equally hard to hit Billy the Baker's Boy or to hit Lady Nonesuch (who has been training at arms since childhood). Bizarre, to say the least.
The other thing which should have a huge effect on your AC, and which doesn't, is your choice of weapon. Look me in the eyes, and tell me with a straight face that it's equally difficult to land a serious blow on a person holding a sword and a person holding nothing. It's farcical. The first job of a melee weapon is to keep you alive. Killing the other person is a secondary concern.
Anyway, this is all a long way of saying that there are two factors to the defense: hard-to-hit and hard-to-hurt.
Not getting hit comprises skill, weapon selection, and natural ability. Lady Nonesuch (who you will recall has been training at arms since childhood), should be very skilled indeed at evading blows, parrying, blocking, ripostes, counterattacks, etc. Even so, if she doesn't have a sword, and Billy the Baker's Boy does, her odds of coming out unscathed don't look so good.
I covered the relative defensive merits of various weapons in my previous post on Weapon Behaviour, and I'll cover skill in an upcoming post about Levelling.
Damage reduction typically comes from armour (on a human), but could also take into account the thick hide of a dragon, the impervious nature of a stone golem, or the difficulty in hacking down a solid oak door. I realized while writing this that insubstantial creature don't fit this model - they should have damage reduction, but it shouldn't be mitigated by using a mace. I'll think about that, and come back to it later - for now, we'll only look at totally corporeal targets.
There were three main types of armour in use in the Middle Ages: cloth, mail, and plate and mail. We can rate them simply as 1, 2, and 3 (with normal clothing being 0) as far as their damage reduction potential. We can also carry on up the scale - I could see a stone golem having an armour rating of 5 or 6 - it's just really hard to make a dent in stone!
|Armour Type||Armour Value|
|None or Normal Clothes||0|
|Cloth or Leather||1|
|Plate and Mail||3|
As an aside, from what I can tell, many of the types of armour commonly listed in D&D are not historical. Studded leather, splint mail, banded mail, scale mail - none of these seem to have historical antecedents (not in medieval Europe, anyway). Early medieval armour consisted of cloth, cloth and mail, or cloth, mail, and plates (either as whole plates, or coats-of-plates/brigandine).
Leather doesn't seem to have been commonly used in Medieval Europe, but it was common elsewhere at the time. From the evidence I've seen, it has generally similar properties to cloth armour.