Armour Class

Armour Class (AC) is a numerical rating that typically ranges from -10 to 10, indicating the level of protection a combatant possesses against rolls to hit. A 10 AC represents a completely unprotected combatant, lacking any form of armour, useful dexterity or tough natural hide. However, even at this level, the mechanics of combat still provide a 50% chance of avoiding a hit when facing a non-leveled attacker. As layers of protection are added, including physical armour, magical enhancements, dexterity bonuses and enchanted protective gear such as cloaks, rings and bracers of protection, the numerical rating of AC drops, improving the character's defensive capability.

The AC value interacts directly with the attacker's THAC0, a number representing the attack roll needed to hit AC 0. To determine the actual number required on a d20 roll to land a hit, subtract the defender's AC from the attacker's THAC0.

For example, Pollux is wearing studded leather +1, with a dexterity of 17 and a cloak of protection +1. Ordinary studded leather provides an AC adjustment of -3, dexterity of 17 reduces AC by another -3, and the magical bonuses from the armour and cloak contribute an additional -2. This results in a final AC of 2. Against Pollux, a 3rd-level fighter with a THAC0 of 18 calculates the chance to hit by subtracting the target's AC from their THAC0 (18 -2 = 16). So they must roll at least a 16 on a d20 to hit Pollux.

When AC falls below zero, calculations follow the same principle. If a defender's AC is -1, then subtracting -1 is equivalent to adding 1 to the THAC0. Using the same example, the fighter with a THAC0 of 18 would now require a 19 on a d20 to hit. If the modified THAC0 reaches 21 or higher, the attacker would be unable to land a hit without additional attack modifiers, rendering the defender effectively invulnerable in a fight.

Complexity

At first glance, the system may seem complicated, particularly as it requires remembering that a +1 bonus to AC is actually subtracted from the combatant's armour class, not added. This reversal of expectation has led to frequent misunderstandings, with numerous players protesting both this apparent inconsistency and the seeming complexity of calculating THAC0. The negative progression of AC can feel counterintuitive at first, as many assume that a higher number should represent greater protection, rather than the opposite.

However, once the system is properly understood and internalised, its simplicity becomes evident. The numerical changes involved are small, with relatively few modifiers applying in most combat scenarios. The THAC0 table is straightforward, with only slight reductions as levels increase, allowing players to commit their character's THAC0 to memory with ease. Once familiar with their AC and THAC0 values, a player can calculate the number needed to hit in 2-3 seconds, making it one of the fastest resolution mechanics in combat. Despite early resistance from those new to the system, experienced players find that THAC0 offers a direct and efficient method for determining hits, eliminating the need for extensive reference tables or complex calculations in the heat of battle.

Special Notes

Some humanoid creatures that are capable of wearing armour possess extraordinarily tough hides, granting them a natural armour class that is significantly better than the standard AC 10 assigned to humans and demi-humans such as dwarves, elves, gnomes, half-elves, halflings and half-orcs. This natural advantage provides creatures like goblins, hobgoblins, bugbears, orcs, ogres and giants with inherent protection that often ranges between AC 7 and AC 2, depending on their species.

If these humanoids wear heavy armour, their AC improves even further, sometimes dramatically. An army of orcs, for example, which have a natural AC of 6, when equipped with chain mail and a shield, would have an adjusted AC of 0 — as difficult to strike as a well-armoured human knight.

Similarly, warhorses and war dogs, despite being non-humanoid, also possess natural armour, typically AC 7 or AC 6. If fitted with leather or chain barding, this additional protection would be subtracted from their base AC, making them much more difficult to wound in battle.

Against large humanoids, those standing over 7 feet tall, both gnomes and dwarves receive a +4 bonus to their AC. This adjustment accounts for their small size and agility in avoiding blows from much taller opponents, whose weapons and attacks are more easily sidestepped by a compact, well-trained fighter.

For monsters, AC is rarely determined by worn armour but rather by natural defenses, including thickened hide, reinforced scales, magical resistance or natural agility in combat. Many creatures possess inherent protective traits that place their AC well above 10, with some reaching negative values due to impenetrable hides or powerful defensive adaptations.

In addition to physical protection, armour class can be affected by situational factors. Conditions such as exposure to chemical attacks (like poison), medical ailments (such as blindness) or instability due to rough terrain can all negatively impact a combatant's AC. Other circumstances, such as rapid movement or attempting to fight while restrained, may further hinder a defender's ability to avoid attacks. These modifiers and adjustments will be fully detailed here at a later time.

See also,
Armour (spell)
Attacking in Combat
Phantom Armour (spell)