Every combatant has an Armor Class, whether he is wearing armor or not. For Player Characters and most humanoid Non-Player Characters, Armor Class is an accurate measure of how much armor that person is wearing. For monsters, Armor Class is a more amorphous reading of size, speed, toughness, supernatural power, and a variety of other factors. An Armor Class of 0 is no protection at all. An Armor Class of 10 is very well protected.
Despite how Armor Class is acquired, it is always a number that helps determine if a target takes damage. How Armor Class is used to determine that depends on whether the damage is caused by an impact or piercing attack.
Impact attacks. The goal of impact attacks is to do damage through armor, instead of bypassing it. For purposes of rolling to hit, all targets are treated as if Armor Class 1 (Dexterity and certain other modifiers to the roll still apply, see Dexterity and Combat). If a hit is scored, damage is rolled, and the Armor Class is subtracted from the damage before the target takes any damage. For example, a target with an Armor Class of 5 is hit for 4 points of damage, counting all modifiers, but after subtracting the Armor Class takes no damage. If the damage had been 6, the target would take 1 point of damage.
Piercing attacks. The goal of piercing attacks is to bypass armor and do damage around it, exploiting any gaps or holes in protection. The attacker’s roll to hit is more difficult, as every modifier is stacked to it, but a hit always means full damage.
Any piercing weapon can be used as an impact weapon for half damage. Certain weapons can be used as an impact or a piercing weapon equally – a dart flung from an atlatl, a crossbow, or any other bow.
Armor bestows cumulative points of protection, so that a helmet, shield, bracers, and greaves can all be worn with any armor. Heavy padding or light hide armor can both be worn under any other armor.
Wearing armor has a negative impact on Dexterity, so that every point of Armor Class worn confers a –1 penalty to maximum Dexterity. So, a human wearing no armor could have a maximum Dexterity of 18, but would have a maximum Dexterity of 16 while wearing heavy hide armor, or a maximum Dexterity of 7 while wearing bracers, greaves, a helm, heavy padded armor, heavy segmented armor, and carrying a shield (the negative modifier for low Dexterity would drop that Armor Class back down to 10). This is not a number subtracted from the Dexterity score itself, so a Dexterity of 18 would become 7, but so would a 14 or a 12.
Additional notes about armor are as follows:
Spangens are open-faced helms. They may or may not have nasal guards.
Light armor is typically a shorter coat, with short sleeves, or with no cuisses (leggings).
Heavy padded armor confers 2 points of Armor Class vs impact weapons only. Light padded armor is merely clothing.
Light scaled armor is also known as studded armor, because the scales do not overlap.
Heavy segmented armor is made of steel bands and is also known as banded armor.
Light segmented armor is typically made of laminated wood.
Shields are typically made in an elongated oval or diamond shape, out of wood bound with iron. Larger or smaller shields, or ones of other composition are not commonly available.