Your skill at fighting with two weapons is unparalleled.
Prerequisites
Dexterity 19, Two-Weapon Fighting, Improved Two-Weapon Fighting, base attack bonus +11.
Benefit
You gain a second iterative attack with an off-hand weapon, allowing you to make three off-hand attacks with that weapon. This third attack is made at a -10 penalty. You must make a full attack to gain the extra attacks.
The penalty for fighting with a weapon in your off hand is reduced by an additional 2 (minimum 0), effectively removing the penalty for both light and one-handed weapons.
The shield bonus to AC gained while wielding two weapons or a double weapon increases by another point to +3 (or to +4 when fighting defensively or using the total defense action).
Special
If you possess the Multiweapon Fighting feat, you can make up to three attack with each off-hand weapon as part of a full attack, taking penalties on the second and third attack as described for fighting with two weapons.