The Attack Roll (DMG)

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 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 various hand-to-hand attacks. Attack rolls are also used to resolve a variety of actions that require accuracy (e.g., 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. Here's a simple example:
:Rath has reached 7th level as a fighter. His THAC0 is 14 (found on Table 38), 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). An 8 or higher on 1d20 will hit the orc. If Rath hits, he rolls the appropriate dice (see Table 44 in the Player's Handbook) to determine how much damage he inflicts.
==Modifiers to the Attack Roll==
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. Figure Strength weapon modifiers, subtract the total from the base THAC0, and record this modified THAC0 for each weapon on the character sheet. Subtract the target's Armor Class from this modified THAC0 when determining the number needed to attack successfully.
Here's the same example, with some common modifiers thrown in:
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 in the Player's Handbook would tell him how much damage he inflicts with his weapon (this information should also be written on his character sheet).
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. 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.
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 35 lists some standard combat modifiers. Positive numbers are bonuses for the attacker; negative numbers are penalties.
The DM can also throw in situational modifiers, (e.g., 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.
 Table 35: 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 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 exist. 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.
===The Various Types of Weapons===
In the AD&D game, weapons fall into several categories, based on how they are used. The three 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 pole arms, 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 weaknesses against certain classes of weapons.
Table 36 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. 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 36: 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 might 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.