Two Weapon Defense

Two-Weapon Defense [General]

Your Two-Weapon fighting style bolsters your defense as well as your offense.

Prerequisites: Dex 15, Two-Weapon Fighting.

Benefit: When wielding a double weapon or two weapons (not including natural weapons or unarmed strikes), you gain a +1 shield bonus to your AC.
When you are fighting defensively or using the total defense action this shield bonus increases to +2.

Special: A fighter may select Two-Weapon Defense as one of his fighter bonus feats (see page 38).

Source: Player's Handbook