Flanking
Two melee allies on opposite sides of a foe make it off-guard (-2 AC) to them. Essential for Rogues and high-MAP turns.
Flanking represents catching an enemy between two threats, making it harder for them to defend.
Requirements: Two allies must be on opposite sides of an enemy (drawing a line through opposite corners or edges of the creature's space) and both must be able to see the target. Both flanking creatures must be wielding melee weapons or threatening with unarmed attacks — a creature with only ranged weapons cannot flank.
Effect: A flanked creature is off-guard (formerly flat-footed) to the flanking creatures, taking a -2 circumstance penalty to AC against their attacks. The flanked creature is NOT off-guard to other attackers — only to the two creatures flanking it.
Flanking and Size: Larger creatures occupy more squares, making flanking geometry easier. A Large creature flanking with an ally just needs them on roughly opposite sides. Tiny creatures may have trouble flanking larger foes.
Tactical Importance: Because off-guard gives -2 AC, flanking effectively provides a +2 bonus to hit. Combined with a Fighter's superior proficiency or a Rogue's Sneak Attack (which requires the target to be off-guard), flanking is one of the most impactful tactical maneuvers in PF2e. Rogues in particular depend on flanking or other sources of off-guard to trigger their key damage feature.