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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.
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 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.
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.
Attacks of Opportunity
The melee combat rules assume that combatants are actively avoiding attacks. A player doesn't have to declare anything special for his or her character to be on the defensive. Sometimes, however, a combatant in a melee lets his or her guard down, and doesn't maintain a defensive posture as usual. In this case, combatants near him or her can take advantage of this lapse in defense to attack for free. These attacks are called attacks of opportunity.
A character can use a melee weapon to make attacks of opportunity whenever the conditions for such an attack are met (see Provoking an Attack of Opportunity, below). In addition, a character can make attacks of opportunity with unarmed attacks if the character's unarmed attacks count as armed (see "Armed" Unarmed Attacks).
A character threatens the squares into which he or she can make a melee attack, even when it is not the character's action. Generally, that's all squares adjacent to the character's position. An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from the character. A character can only make attacks of opportunity with melee weapons, never with ranged weapons.
Provoking an Attack of Opportunity
Two actions can provoke attacks of opportunity: moving out of a threatened square, and performing an action within a threatened square that distracts from defending and lets the character's guard down.
Moving out of a Threatened Square: When a character moves out of a threatened square, that character generally provokes an attack of opportunity. There are two important exceptions, however.
A character doesn't provoke an attack of opportunity if all he or she moves is a 5-foot step, or if the character withdraws.
If the character doesn't start in a threatened square, but moves into one, the character has to stop there, or else he or she provokes an attack of opportunity as he or she leaves that square.
Performing an Action that Distracts the Character: Some actions, when performed in a threatened square, provoke attacks of opportunity because they make a character divert his or her attention from the fight at hand. Using a ranged weapon, in particular, provokes attacks of opportunity. Table: Actions in Combat notes many additional actions that provoke attacks of opportunity.
Making an Attack of Opportunity
An attack of opportunity is a single melee attack, and a character can only make one per round. A character does not have to make an attack of opportunity if he or she doesn't want to.
An experienced character gets additional regular melee attacks (by using the full attack action), but at a lower attack bonus. A character makes his or her attack of opportunity, however, at his or her normal attack bonus-even if the character has already attacked in this round.