Flanking

If an ally of yours is on the square on the opposite side of the opponent, you deal 1.5x damage. This also applies if the ally is not engaged in combat or busy with another enemy.

Example: The rangers Bido and Bodo, both equipped with daggers, surround an evil troll, with Bido on one side and Bodo on the other. Bido attacks and rolls 1d4. With his +3 in Dexterity he deals 5 damage, but since the troll is flanked, he instead deals 5 * 1.5 = 7 (rounded down) damage.