You can fight with a weapon wielded in each of your hands. You can make one extra attack each round with the secondary weapon.
Prerequisite: Dex 15.
Benefit: Your penalties on attack rolls for fighting with two weapons are reduced. The penalty for your primary hand lessens by 2 and the one for your off hand lessens by 6. See Two-Weapon Fighting.
Normal: If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. When fighting in this way you suffer a –6 penalty with your regular attack or attacks with your primary hand and a –10 penalty to the attack with your off hand. If your off-hand weapon is light, the penalties are reduced by 2 each. An unarmed strike is always considered light.
If you use Two-Weapon Fighting on your turn to attack with two weapons, do you also take that penalty on attacks of opportunity made before the start of your next turn? How long do the penalties last?
No, the penalties end as soon as you have completed the full-attack action that allowed you to attack with both weapons. Any attacks of opportunity you make are at your normal attack bonus. Generally speaking, penalties on attacks made during your turn do not carry over to attacks of opportunity unless they specifically state otherwise (such as the penalty from using Power Attack).
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.