Combat rules are based on the use of dice rolls. Each time an action is performed in combat, the game simulates the rolling of one or more dice to determine the result. The dice roll is referred to by a code in the form: number of dice, followed by “d,” followed by a numeral for the type of dice. For example, one 6-sided die is "1d6"; five 12-sided dice are “5d12.”
- Main article: Armor Class
Armor Class (AC) is the protective rating of a type of armor. Armor provides protection by reducing the chance that a character is attacked successfully (and suffers damage). Armor does not absorb damage, it prevents it. Penetrating the armor of a fighter in full plate mail to cause any damage is no small task.
Armor Class is measured on a scale from 10, the worst (no armor), to –10, the best (very powerful magical armors). The lower the number, the more effective the armor. Shields can also improve the AC of a character.
High dexterity gives a bonus to Armor Class.
- Main article: Damage
Damage is what happens to a character when an opponent attacks him successfully. Damage can also occur as a result of poison, fire, acid and other elemental damage types, and anything even remotely dangerous in the real world. Damage from most attacks is measured in hit points. Each time a character is hit, he suffers points of damage. It can be as little as 1 point to as many as 80 or more. These points are subtracted from the character’s current hit point total. When this reaches 0, the character is dead.
Missile combat is defined as any time a weapon is shot, thrown, hurled, or otherwise propelled. Missile and melee combat have the same basic rules, but there are special situations and modifiers that apply only to missile combat; a character's dexterity, for example, affects their chance to hit.
- Main article: THAC0
THAC0 is an acronym for “To Hit Armor Class 0.” This is the number a character, NPC, or monster needs to attack an Armor Class 0 target successfully. THAC0 depends on a character’s level. The THAC0 number is used to calculate the number needed to hit any Armor Class. THAC0 improves as the level of a character increases.
- Main article: Saving throw
Saving throws are measures of a character’s resistance to special types of attacks – poisons, magic, and attacks that affect the whole body or mind of the character. The ability to make successful saving throws improves as the character increases in level.
- Main article: Speed Factor
Speed factors are numbers between 1 and 10. The lower the speed factor of a weapon, the more quickly the character wielding that weapon begins their sequence of attacks.
The casting time of a spell is the amount of time the caster is busy doing the motions of casting before the spell’s effects take place. Only one spell per round can be cast. Even if you cast a spell with a short casting time, you must wait until the end of the round before starting your next spell.
The attack rollEdit
An attack roll is a simulated twenty sided die roll (1d20) that determines whether an attack succeeds or fails. The attack is successful if the result of the die roll is equal or higher than a certain number. This number is called the “to-hit” number.
The to-hit numberEdit
Example: Rath is a 7th level fighter. His THAC0 is 14, which means that he needs to roll a 14 or better to hit a character or creature of Armor Class 0. In combat, Rath, attacking an orc wearing chainmail armor (AC 6), needs to roll an 8 (14–6=8) to hit the orc. An 8 or higher on 1d20 will hit the orc. If Rath hits, he rolls the appropriate dice to determine how much damage he inflicts. 
In a typical game combat situation, THAC0 is modified by weapon bonuses, strength bonuses, and the like. The actual to-hit number is the result of subtracting the target’s Armor Class from this modified THAC0.
Example: Rath is still a 7th-level fighter. He has a Strength of 18/80, which gives him a +2 bonus to his attack roll. He fights with a long sword +1. His base THAC0 is 14, which is modified to 12 by his Strength and to 11 by his weapon. If attacking the orc from the earlier example, Rath would have to roll a 5 or higher on 1d20 in order to hit (11–6=5). 
If the die roll on 1d20 is equal to or greater than this adjusted to-hit number, the attack succeeds. If the roll is lower, the attack fails.
Modifiers to the to-hit numberEdit
Many factors can modify the number a character needs for a successful hit. These variables are called modifiers to the to-hit number or to the attack roll.
Strength Modifier: A character’s strength can modify the to-hit number, altering both the chance to hit and the damage caused. This modifier is applied to melee attacks.
Magical items: The magical properties of a weapon also attack rolls. Items that give a bonus to the attack roll or Armor Class often are identified by a plus sign. For example, a sword +1 may improve a character’s to-hit number by one. A suit of chain mail +1 may improve the Armor Class of the character by one over the protection provided by a non-magical suit of chain mail. (In this case, the magical version has an AC of 4 while the non-magical one has an AC of 5). Cursed items may have a negative modifier (a penalty), resulting in a subtraction from the attack roll or an addition to Armor Class.
There is no limit to the number of modifiers that can be applied to the to-hit number or the AC, nor is there a limit to the positive or negative number (the total of all modifiers).
If the target is sleeping or held, the attack automatically hits and causes normal damage.