|d20 Modern System Reference Document|
Combat is played out in rounds, and in each round everybody acts in turn in a regular cycle. Combat usually runs in the following way.
An attack roll represents a character's attempts to strike an opponent on the character's turn in a round. When a character makes an attack roll, he or she rolls 1d20 and adds his or her attack bonus. If the result equals or beats the target's Defense, the character hits and deals damage. Many modifiers can affect the attack roll.
A natural 1 (the d20 comes up 1) on the attack roll is always a miss. A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also always a threat--a possible critical hit.
If the character is not proficient in the weapon he or she is attacking with (the character doesn't have the appropriate Weapon Proficiency feat), that character takes a –4 penalty on the attack roll.
A character's attack bonus with a melee weapon is:
Base attack bonus + Strength modifier + size modifier
With a ranged weapon, a character's attack bonus is:
Base attack bonus + Dexterity modifier + range penalty + size modifier
Strength Modifier: Strength helps a character swing a weapon harder and faster, so a character's Strength modifier applies to melee attack rolls.
Size Modifier: Creature size categories are defined differently from the size categories for weapons and other objects. Since this size modifier applies to Defense against a melee weapon attack or a ranged weapon attack, two creatures of the same size strike each other normally, regardless of what size they actually are. Creature sizes are compatible with vehicle sizes.
Dexterity Modifier: Dexterity measures coordination and steadiness, so a character's Dexterity modifier applies when the character attacks with a ranged weapon.
Range Penalty: The range penalty for a ranged weapon depends on what weapon the character is using and how far away the target is. All ranged weapons and thrown weapons have a range increment (see Table: Ranged Weapons and Table: Melee Weapons). Any attack from a distance of less than one range increment is not penalized for range. However, each full range increment causes a cumulative –2 penalty on the attack roll. A thrown weapon has a maximum range of five range increments. Ranged weapons that fire projectiles can shoot up to ten increments.
When a character hits with a weapon, he or she deals damage according to the type of weapon. Effects that modify weapon damage also apply to unarmed strikes and the natural physical attack forms of creatures.
Damage is deducted from the target's current hit points.
Minimum Weapon Damage: If penalties to damage bring the damage result below 1, a hit still deals 1 point of damage.
Strength Bonus: When a character hits with a melee weapon or thrown weapon, add his or her Strength modifier to the damage.
Off-Hand Weapon: When a character deals damage with a weapon in his or her off hand, add only half of the character's Strength bonus.
Wielding a Weapon Two-Handed: When a character deals damage with a weapon that he or she is wielding two-handed, add 1.5 times the character's Strength bonus. However, the character doesn't get this higher Strength bonus when using a light weapon two-handed; in such a case, only the character's normal Strength bonus applies to the damage roll.
Sometimes damage is multiplied by some factor. Roll the damage (with all modifiers) multiple times and total the results.
Bonus damage represented as extra dice is an exception. Do not multiply bonus damage dice when a character scores a critical hit.
A critical hit multiplies the character's damage. Unless otherwise specified, the multiplier is x2. (It is possible for some weapons to have higher multipliers, doing more damage on a critical hit.) Some weapons have expanded threat ranges, making a critical hit more likely. However, even with these weapons, only a 20 is an automatic hit. The Critical column on Table: Ranged Weapons and Table: Melee Weapons indicates the threat range for each weapon on the tables.
Bonus damage represented as extra dice is not multiplied when a character scores a critical hit.
Objects (including vehicles) and some types of creatures are immune to critical hits. A 20 is always a successful hit, but deals no extra damage against these targets.
A character's Defense represents how hard it is for opponents to land a solid, damaging blow on the character. It's the attack roll result that an opponent needs to achieve to hit the character. The average, unarmored civilian has a Defense of 10. A character's Defense is equal to:
10 + Dexterity modifier + class bonus + equipment bonus + size modifier
Dexterity Modifier: If a character's Dexterity is high, he or she is particularly adept at dodging blows or gunfire. If a character's Dexterity is low, he or she is particularly inept at it. Characters apply their Dexterity modifier to Defense.
Sometimes a character can't use his or her Dexterity bonus. If a character can't react to a blow, that character can't use his or her Dexterity bonus to Defense.
Class Bonus: A character's class and level grant the character an innate bonus to Defense. This bonus applies in all situations, even when the character is flat-footed or when the character would lose his or her Dexterity bonus for some other reason.
Equipment Bonus: If a character wears armor, it provides a bonus to his or her Defense. This bonus represents the armor's ability to protect the character from blows.
Armor provides a minimum bonus to anyone who wears it, but a character who is proficient in the use of a certain type of armor receives a larger bonus to Defense.
Sometimes a character can't use his or her equipment bonus to Defense. If an attack will damage the character just by touching him or her, that character can't add his or her equipment bonus (see Touch Attacks, below).
Size Modifier: The bigger an opponent is, the easier it is to hit in combat. The smaller it is, the harder it is to hit. Since this same modifier applies to attack rolls a creature doesn't have a hard time attacking another creature of the same size. Size modifiers are shown on Table: Size Modifiers.
Other Modifiers: Other factors can add to a character's Defense.
Feats: Some feats give a bonus to a character's Defense.
Natural Armor: Some creatures have natural armor, which usually consists of scales, fur, or layers of thick muscle.
Dodge Bonuses: Some other Defense bonuses represent actively avoiding blows. These bonuses are called dodge bonuses. Any situation that denies a character his or her Dexterity bonus also denies his or her dodge bonuses. Unlike most sorts of bonuses, dodge bonuses stack with each other.
Magical Effects: Some campaigns may include magic. Some magical effects offer enhancement bonuses to armor (making it more effective) or deflection bonuses that ward off attacks.
Some attacks disregard armor. In these cases, the attacker makes a touch attack roll (either a ranged touch attack roll or a melee touch attack roll). The attacker makes his or her attack roll as normal, but a character's Defense does not include any equipment bonus or armor bonus. All other modifiers, such as class bonus, Dexterity modifier, and size modifier, apply normally.
A character's hit points tell how much punishment he or she can take before dropping. Hit points are based on the character's class and level, and the character's Constitution modifier applies.
When a character's hit point total drops to 0, he or she is disabled. When it drops to –1, he or she is dying. When it drops to –10, the character is dead.
A character's speed tells how far he or she can move in a move action. Humans normally move 30 feet, but some creatures move faster or slower. Wearing armor can slow a character down.
A character normally moves as a move action, leaving an attack action to attack. The character can, however, use his or her attack action as a second move action. This could let the character move again, for a total movement of up to double his or her normal speed. Another option is to run all out (a full-round action). This lets the character move up to four times his or her normal speed, but a character can only run all out in a straight line, and doing so affects the character's Defense (see Run).
Generally, when a character is subject to an unusual or magical attack, he or she gets a saving throw to avoid or reduce the effect. A saving throw is a 1d20 roll plus a bonus based on the character's class and level (the character's base save bonus) and an ability modifier.
A natural 1 (the d20 comes up 1) on a saving throw is always a failure. A natural 20 (the d20 comes up 20) is always a success.
A character's saving throw bonus is:
Base attack bonus + Strength modifier + size modifier
The Difficulty Class for a save is determined by the attack itself.
Saving Throw Types
The three different kinds of saving throws are:
Fortitude: These saves measure a character's ability to stand up to massive physical punishment or attacks against his or her vitality and health such as poison and paralysis. Apply a character's Constitution modifier to his or her Fortitude saving throws.
Reflex: These saves test a character's ability to dodge massive attacks such as explosions or car wrecks. (Often, when damage is inevitable, a character gets to make a Reflex save to take only half damage.) Apply the character's Dexterity modifier to his or her Reflex saving throws.
Will: These saves reflect a character's resistance to mental influence and domination as well as to many magical effects. Apply the character's Wisdom modifier to his or her Will saving throws.
Every round, each combatant gets to do something. The combatants' initiative checks, from highest to lowest, determine the order in which they act, from first to last.
Initiative Checks: At the start of a battle, each combatant makes a single initiative check. An initiative check is a Dexterity check. Each character applies his or her Dexterity modifier to the roll, and anyone with the Improved Initiative feat gets an additional +4 bonus on the check. The GM finds out what order characters are acting in, counting down from highest result to lowest, and each character acts in turn. On all following rounds, the characters act in the same order (unless a character takes an action that results in his or her initiative changing; see Special Initiative Actions). If two or more combatants have the same initiative check result, the combatants who are tied go in order of total initiative modifier (including Dexterity modifier and Improved Initiative bonus, if applicable). If there is still a tie, roll a die.
Flat-Footed: At the start of a battle, before the character has had a chance to act (specifically, before the character's first turn in the initiative order), the character is flat-footed. A character can't use his or her Dexterity bonus to Defense or make attacks of opportunity while flat-footed.
Joining a Battle: If characters enter a battle after it has begun, they roll initiative at that time and act whenever their turn comes up in the existing order.
When a combat starts, if a character was not aware of his or her enemies and they were aware of the character, that character is surprised. Likewise, a character can surprise his or her enemies if the character knows about them before they're aware of the character.
The Surprise Round: If some but not all of the combatants are aware of their opponents, a surprise round happens before regular rounds begin. The combatants who are aware of the opponents can act in the surprise round, so they roll for initiative. In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take an attack action or move action during the surprise round (see Action Types, below). If no one or everyone is surprised, a surprise round does not occur.
Unaware Combatants: Combatants who are unaware at the start of battle do not get to act in the surprise round. Unaware combatants are still flat-footed because they have not acted yet. Because of this, they lose any Dexterity bonus to Defense.