Combat and thac0

[Dungeon Master]

Definitions
Many game terms are used throughout the combat rules. To understand the rules, players must understand these terms, so brief explanations appear below. Further details are provided throughout this chapter.
Armor Class (AC) is the protective rating of a type of armor. In some circumstances, AC is modified by 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 AC.
Armor provides protection by reducing the chance that a character is attacked successfully (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 his 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 can also improve the AC of a character (see "Shields" in Chapter 6: Money and Equipment).
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 is attacked from the rear.
Damage is what happens to a character when an opponent attacks him successfully. 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 by 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 die 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 successful saving throws improves as the character increases in 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 (see Table 51). A surprised character also has a decreased chance of rolling a successful saving throw, if one is needed.
Surprise 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 successfully. THAC0 depends on a character's group and level (see Table 53). 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 successful 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 (for example, throwing a rock at a small target or tossing a sword to a party member in the middle of a fight).

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.

Rath has reached 7th level as a fighter. His THAC0 is 14 (found on Table 53), meaning 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 (see Table 44) to determine how much damage he inflicts.

The example above is quite simple--in a typical AD&D game combat situation, THAC0 is modified by weapon bonuses, Strength bonuses, and the like (the next section "Modifiers to the Attack Roll," lists the specifics of these modifiers). Figure Strength and weapon modifiers, subtract the total from the base THAC0, and record this modified THAC0 with each weapon on the character sheet. Subtract the target's Armor Class from this modified THAC0 when determining the to-hit number.

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 THAC0 is 14, 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). Again, table 44 would tell him how much damage he inflicts with his weapon (this information should also be written on his character sheet).

The DM may also throw in situational modifiers, (for example, a bonus if the target is struck from behind, or a penalty if the target is crouching behind a boulder). If the final, modified die roll on 1d20 is equal to 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 successful 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 spear or an axe).
A positive Strength modifier can be applied to bows if the character has a special bow made for him, 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. For example, a sword +1 improves a character's chance to hit by one. A suit of chain mail +1 improves the Armor Class of the character by one (which means you subtract one from the character's AC, changing an AC of 5 to an AC of 4, for example). 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 modifiers) that can be applied to a die roll.
Table 51 lists some standard combat modifiers. Positive numbers are bonuses for the attacker; negative numbers are penalties.

Table 51:
Combat Modifiers

Attack Roll
Situation 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 attack +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
(Optional Rule)
Not all weapons perform the same. If they did, there would be no need for the wide variety of weapons that exists. Only one form of each weapon type, the most useful one, would be used throughout the world. This is obviously not the case.
Aside from the differences in size, weight, length, and shape, certain types of weapons are more useful against some types of armor than others. Indeed, the different armors and weapons of the world are the result of an ancient arms race. Every new weapon led to the development of a new type of armor designed to counter it. This led to new weapons, which led to new armor, and so on.
In the AD&D game, weapons fall into several categories, based on how they are used. The basic categories are slashing, piercing, and bludgeoning.
Slashing weapons include swords, axes, and knives. Damage is caused by the combination of weight, muscle, and a good sharp edge.
Piercing weapons (some swords, spears, pikes, arrows, javelins, etc.) rely on the penetrating power of a single sharp point and much less on the weight of the weapon.
Bludgeoning weapons (maces, hammers, and flails) depend almost entirely on the impact caused by weight and muscle.
A few weapons, particularly some of the more exotic polearms, fall into more than one of these categories. A halberd can be used as a pole-axe (a slashing weapon) or as a short pike (a piercing weapon). The versatility of these weapons provides the user with a combat advantage, in that the mode most favorable to the attacker can be used, depending upon the situation.
Natural weapons can also be classified according to their attack type. Claws are slashing weapons; a bite pierces; a tail attack bludgeons. The DM must decide which is most appropriate to the creature and method of attack.
Armor types, in turn, have different qualities. Field plate is more effective, overall, than other armors by virtue of the amount and thickness of the metal, but it still has specific weaknesses against certain classes of weapons.
Table 52 lists the weapon vs. armor modifiers applied to the attacker's THAC0, if this optional system is used. To use this table, the actual armor type of the target must be known in addition to the target's Armor Class. The bonuses of magical armor do not change the type of armor, only the final Armor Class.
This system is used only when attacking creatures in armor. The modifiers are not used when attacking creatures with a natural Armor Class.

Table 52:
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 +4 +3 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 plate 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 ridiculously 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.
Thus, even if a character's chance to hit a monster is 23 and the character has a -3 penalty applied to the die roll, he might be able to score a hit--but only if the die roll is a 20 before any modifiers are applied. Likewise, a character able to hit a monster on a 3 or better, waving a sword +4, could still miss if a 1 is rolled on the die.
There are no sure things, good or bad, in the unpredictable chaos of combat situations.

Table 53:
CALCULATED THAC0S

Level
Group 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

Table 54:
THAC0 Advancement

Improvement Rate
Group 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 character 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.
For a character of level 1 through level 20, consult Table 53. This table lists the THAC0 number of each group through 20th level, so players don't have to perform any calculations.
For a character higher than 20th level, find the Improvement Rate for the character's group in Table 54. There you'll find the number of levels a character must advance to reduce his THAC0 by 1 (or more) points. Calculate the character's THAC0 according to his level.
The DMG contains the information on monster THAC0s.

Combat and Encounters
Encounters are the heart of the AD&D game. Since encounters with monsters and NPCs often lead to combat, an understanding of what happens during battles is vital for everyone. There are several factors the DM will consider in any combat, most of which arise from the circumstances of the encounter. Is anyone surprised? How far apart are the opponents? How many of them are there? Answers to these questions are found in the Encounter section of the DMG. These are questions common to all encounters, whether combat occurs or not.

[Dungeon Master]

The Combat Round
If an encounter escalates into a combat situation, the time scale of the game automatically goes to rounds (also called melee rounds or combat rounds). Rounds are used to measure the actions of characters in combat (or other intensive actions in which time is important).
A round is approximately one minute long. Ten combat rounds equal a turn (or, put another way, a turn equals 10 minutes of game time). This is particularly important to remember for spells that last for turns, rather than rounds.
But these are just approximations--precise time measurements are impossible to make in combat. An action that might be ridiculously easy under normal circumstances could become an undertaking of truly heroic scale when attempted in the middle of a furious, chaotic battle.
Imagine the simple act of imbibing a healing potion. First, a character decides to drink the potion before retiring for the night. All he has to do is get it out of his backpack, uncork it, and drink the contents. No problem.
Now imagine the same thing in the middle of a fight. The potion is safely stowed in the character's backpack. First, he takes stock of the situation to see if anyone else can get the potion out for him, but, not surprisingly, everyone is rather busy. So, sword in one hand, he shrugs one strap of the pack off his shoulder. Then, just as two orcs leap toward him, the other strap threatens to slip down, entangling his sword arm. Already the loose strap keeps him from fully using his shield.
Holding the shield as best as possible in front of him, he scrambles backward to avoid the monsters' first wild swings. He gets pushed back a few more feet when a companion shoulders past to block their advance. His companion bought him a little time, so he kneels, lays down his sword, and slips the backpack all the way off. Hearing a wild cry, he instinctively swings his shield up just in time to ward off a glancing blow.
Rummaging through the pack, he finally finds the potion, pulls it out, and, huddling behind his shield, works the cork free. Just then there is a flash of flame all around him--a fireball! He grits his teeth against the heat, shock, and pain and tries to remember not to crush or spill the potion vial. Biting back the pain of the flames, he is relieved to see the potion is still intact.
Quickly, he gulps it down, reclaims his sword, kicks his backpack out of the way, and runs back up to the front line. In game terms, the character withdrew, was missed by one attacker, made a successful saving throw vs. spell (from the fireball), drank a potion, and was ready for combat the next round.

What You Can Do in One Round
Whatever the precise length of a combat round, a character can accomplish only one basic action in that round, be it making an attack, casting a spell, drinking a potion, or tending to a fallen comrade. The basic action, however, may involve several lesser actions.
When making an attack, a character is likely to close with his opponent, circle for an opening, feint here, jab there, block a thrust, leap back, and perhaps finally make a telling blow. A spellcaster may fumble for his components, dodge an attacker, mentally review the steps of the spell, intone the spell, and then move to safety when it is all done. It has already been shown what drinking a potion might entail. All of these things might happen in a bit less than a minute or a bit more, but the standard is one minute and one action to the round.
Some examples of the actions a character can accomplish include the following:
Make an attack (make attack rolls up to the maximum number allowed the character class at a given level)
• Cast one spell (if the casting time is one round or less)
• Drink a potion
• Light a torch
• Use a magical item
• Move to the limit of his movement rate
• Attempt to open a stuck or secret door
• Bind a character's wounds
• Search a body
• Hammer in a spike
• Recover a dropped weapon

There are also actions that take a negligible amount of time, things the character does without affecting his ability to perform a more important task. Examples of these include the following:
• Shout warnings, brief instructions, or demands for surrender, but not conversations where a reply is expected.
• Change weapons by dropping one and drawing another.
• Drop excess equipment, such as backpacks, lanterns, or torches.

The Combat Sequence
In real life, combat is one of the closest things to pure anarchy. Each side is attempting to harm the other, essentially causing disorder and chaos. Thus, combats are filled with unknowns--unplanned events, failed attacks, lack of communication, and general confusion and uncertainty. However, to play a battle in the game, it is necessary to impose some order on the actions that occur. Within a combat round, there is a set series of steps that must be followed. These steps are:

1. The DM decides what actions the monsters or NPCs will take, including casting spells (if any).
2. The players indicate what their characters will do, including casting spells (if any).
3. Initiative is determined.
4. Attacks are made in order of initiative.

These steps are followed until the combat ends--either one side is defeated, surrenders, or runs away.
NPC/Monster Determination: In the first step, the DM secretly decides in general terms what each opponent will do--attack, flee, or cast a spell. He does not announce his decisions to the players. If a spell is to be cast, the DM picks the spell before the players announce their characters' actions.
Player Determination: Next, the players give a general indication of what their characters are planning to do. This does not have to be perfectly precise and can be changed somewhat, if the DM decides circumstances warrant.
If the characters are battling goblins, a player can say, "My fighter will attack" without having to announce which goblin he will strike. If the characters are battling a mixed group of goblins and ogres, the player has to state whether his character is attacking goblins or ogres.
Spells to be cast must also be announced at this time and cannot be changed once the initiative die is rolled.
Before moving on, the DM will make sure he has a clear idea of not only what the player characters are doing, but also what actions any hirelings and henchmen are taking. Once he has a clear view of everything that's likely to happen, the DM can overrule any announced action that violates the rules (or in the case of an NPC, is out of character).
He is not required to overrule an impossible action, but he can let a character attempt it anyway, knowing full well the character cannot succeed. It is not the DM's position to advise players on the best strategies, most intelligent actions, or optimum maneuvers for their characters.
Initiative: In the third step, dice are rolled to determine initiative, according to the rules for initiative (see "Initiative" below).
Resolution: In the last step, PCs, NPCs, and monsters make their attacks, spells occur, and any other actions are resolved according to the order of initiative.
The above sequence is not immutable. Indeed, some monsters violate the standard sequence, and some situations demand the application of common sense. In these cases the DM's word is final.