Flanking

When you and an ally are flanking a foe, it has a harder time defending against you. A creature is flat-footed (granting attackers +1d to hit) to melee attacks from creatures that are flanking it.

To flank a foe, you and your ally must be on opposite sides of the creature. A line drawn between the center of your space and the center of your ally’s space must pass through opposite sides or opposite corners of the foe’s space. A large or larger creature is flanking as long as at least one square of its space qualifies for flanking.

Additionally, both you and the ally have to be able to act and see the creature. You must be wielding melee weapons, you can’t be under any effects that prevent you from attacking and you must both have the enemy within reach. If you are wielding a reach weapon, you can use your reach with that weapon for this purpose.

A flanked creature may choose one of the attackers that are flanking it and deny them the benefit of flanking. However, all other attackers receive an additional +1d. The chosen attacker may be changed as a free action on the creature’s turn.