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.

If you have base attack bonus of +6 or higher, you instead gain +2 shield bonus to your AC, and +4 when fighting defensively or using the total defense action.

If you have base attack bonus of +11 or higher, you instead gain +3 shield bonus to your AC, and +6 when fighting defensively or using the total defense action.

If you have base attack bonus of +16 or higher, you instead gain +4 shield bonus to your AC, and +8 when fighting defensively or using the total defense action.