Chink in the Armor( Song and Silence: A Guidebook to Bards and Rogues, p. 38)
You are an expert at slipping a weapon between armor plates or into seams.
Combat Expertise (PH) ,
If you take a standard action to study an opponent, you can ignore half of his or her armor bonus (rounded down) during your next single attack. Only bonuses from actual armor (including natural armor) are halved, not those from shields, enhancement bonuses to armor, or magic items that provide an armor bonus.