Definitions

Armor Class (AC) is the protective rating of a type of armor. In some
circumstances, AC is modified by the the amount of protection gained or lost
because of the character's situation. For instance, crouching behind a boulder
improves a character's Armor Class, while being attacked from behind worsens
his/her AC.
Armor provides protection by reducing the chance that a character is attacked
succesfully (and suffers damage). Armor does not absorb damage, it prevents it.
A fighter in full plate mail may be a slow-moving target, but penetrating the
armor 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 and helmets can also improve the AC of a character.
Abilities and situations can also affect a character's Armor Class. High
Dexterity gives a bonus to Armor Class, for example. But even a character with
a Dexterity bonus can have this bonus negated if he/she's attacked from the
rear.
Damage is what happens to a character when an opponent attacks him succesfully.
Damage can also occur as a result of poison, fire, falling, acid, 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
could 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.
Initiative determines the order in which things happen in a combat round. Like
so many things in the world, initiative is determined of a combination of
ability, situation and chance.
At the start of each round of a battle, an initiative roll is made by both 
sides. This roll can be modified by the abilities of the combatants and by the
situation. The person or side with the lower modified roll acts first.
Melee is any situation in which characters are battling each other hand-to-hand,
whether with fists, teeth, claws, swords, axes, pikes, or something else. 
Strength and Dexterity are valuable assets in melee.
Missile combat is defined as any time a weapon is shot, thrown, hurled, kicked,
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.
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 succesful saving throws improves as the character
increases level; Dexterity and general mental fortitude aid in honing combat
senses. Experience makes saving throws easier.
Surprise can happen any time characters meet another group unexpectedly 
(monsters, evil knights, peasants, etc.). Surprise is simply what happens when
one side - a person or party - is taken unawares, unable to react until they
gather their wits. Their opponents, if unsurprised, are allowed a bonus round of
action while the surprised characters recover. It's entirely possible for both
sides in a given situation to be surprised!
Attacking with surprise gives bonuses to the attack roll. A surprised character
also has a decreased chance of rolling a succesful saving throw, if one is
needed.
Surpise is determined by a die roll and is normally checked at the beginning of
an encounter. Surprise is very unpredictable, so there are very few modifiers to
the roll.
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 succesfully. THAC0
depends on a character's group and level. The THAC0 number can be used to
calculate the number needed to hit any Armor Class. THAC0 is refigured each time
a character increases in level. Using THAC0 speeds the play of combat greatly.

The Attack Roll

At the heart of the combat system is the attack roll. This is the die roll that
determines whether an attack succeeds or fails. The number a player needs in
order to make a succesful attack roll is also called the "to-hit" number.
Attack rolls are used for attacks with swords, bows, rocks, and other weapons,
as well as blows from fists, tackling, and other hand-to-hand attacks. Attack
rolls are also used to resolve a variety of potentially injury-causing actions
that require accuracy.

Figuring the To-Hit Number

The first step in making an attack roll is to find the number needed to hit the
target. Subtract the Armor Class of the target from the attacker's THAC0. 
(Remember that if the Armor Class is a negative number, you add it to the
attacker's THAC0.) The character has to roll the resulting number, or higher, on
1d20 to hit the target.
In a typical AD&D(r) game combat situation, THAC0 is modified by weapon bonuses,
Strength bonuses, and the like. Figure Strength and weapon modifiers, subtract
the total from the base THAC0, and record this modified THAC0 which each weapon
on the character sheet. Subtract the target's Armor Class from this modified
THAC0 when determining the to-hit number.
The DM may also throw in situational modifiers. If the final, modified die roll
on 1d20 is equal or greater than the number needed to hit the target, the attack
succeeds. If the roll is lower than that needed, the attack fails.

Modifiers to the Attack Roll

In combat, many factors can modify the number a character needs for a succesful
hit. These variables are reflected in modifiers to the to-hit number or to the
attack roll.
Strength Modifiers: A character's Strength can modify the die roll, altering
both the chance to hit and the damage caused. This modifier is always applied
to melees and attacks with hurled missile weapons.
A positive Strength modifier can be applied to bows if the character has a
special bow made for him/her, designed to take advantage of his high Strength.
Characters with Strength penalties always suffer them when using a bow weapon.
They simply are not able to draw back the bowstring far enough. Characters never
have Strength modifiers when using crossbows - the power of the shot is imparted
by a machine, not the player character.
Magical items: The magical properties of a weapon can also modify combat. Items
that impart a bonus to the attack roll or Armor Class are identified by a plus
sign. Cursed items 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 a single
die roll. Nor is there a limit to the positive or negative number (the total of
all the modifiers) that can be applied to a die roll.
Positive numbers are bonuses for the attacker; negative numbers are penalties.

COMBAT MODIFIERS

Situation			Attack Roll Modifier
Attacker on higher ground	+1
Defender invisible		-4
Defender off-balance		+2
Defender sleeping or held	Automatic*
Defender stunned or prone	+4
Defender surprised		+1
Missile fire, long range	-5
Missile fire, medium range	-2
Rear atack			+2

*If the defender is attacked during the course of a normal melee, the attack
automatically hits and causes normal damage. If no other fighting is going on
(i.e., all others have been slain or driven off), the defender can be slain
automatically.

WEAPON TYPE VS. ARMOR MODIFIERS

Armor Type	Slash	Pierce	Bludgeon
Banded mail	-2	0	-1
Brigandine	-1	-1	0
Chain mail*	-2	0	+2
Field Plate	-3	-1	0
Full Plate	-3	-1	0
Leather armor**	0	+2	0
Plate mail	-3	0	0
Ring mail	-1	-1	0
Scale mail	0	-1	0
Splint mail	0	-1	-2
Studded leather	-2	-1	0

*Includes bronze chain mail
**Includes padded armor and hides

Impossible To-Hit numbers

Sometimes the attacker's to-hit number seems impossible to roll. An attack may
be so difficult it requires a roll greater than 20 (on a 20-sided die!), or so
rediculously easy it can be made on a roll less than 1. In both cases, an attack
roll is still required!
The reason is simple: With positive die roll modifiers (for magic, Strength,
situation, or whatever), a number greater than 20 can be rolled. Likewise, die
roll penalties can push the attack roll below 0.
No matter what number a character needs to hit, a roll of 20 is always
considered a hit and a roll of 1 is always a miss, unless the DM rules
otherwise. Under most circumstances, a natural 20 hits and a natural 1 misses,
regardless of any modifiers applied to the die roll.
There are no sure things, good or bad, in the unpredictable chaos of combat
situations.

CALCULATED THAC0S

Group	Level
	1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17 18 19 20
Priest	20 20 20 18 18 18 16 16 16 14 14 14 12 12 12 10 10 10 8  8
Rogue	20 20 19 19 18 18 17 17 16 16 15 15 14 14 13 13 12 12 11 11
Warrior 20 19 18 17 16 15 14 13 12 11 10 9  8  7  6  5  4  3  2  1
Wizard  20 20 20 19 19 19 18 18 18 17 17 17 16 16 16 15 15 15 14 14

THAC0 ADVANCEMENT

Group	Improvement Rate Points/Level
Priest	2/3
Rogue	1/2
Warrior	1/1
Wizard	1/3

Calculating THAC0

To make an attack roll, the character's THAC0 must be known. This depends on the
group and level, if the attacker is a player or NPC, or the Hit Dice if the
attacker is a monster or an animal. All 1st-level characters have THAC0s of 20,
regardless of class.
