Determining Cover for Melee Attacks
Defender’s Burden: The target of a melee attack has to prove that it has cover. That proof consists of a line between the attacker and the defender that is blocked by a solid object.
Corner to Corner: The defender has cover if an imaginary line from a corner of the attacker’s space to a corner of the defender’s space is blocked.
Getting Technical: If you need to be extremely precise, choose a square the attacker occupies and a square the defender occupies. Draw an imaginary line from every corner of the attacker’s space to every corner of the defender’s space. If even one line is obstructed, the defender has cover. (A line hat runs parallel right along a wall isn’t blocked.)
Superior Cover: Only specific terrain features (such as grates and arrow slits) grant superior cover from melee attacks.
Published in Dungeon Master's Guide, page(s) 43, Rules Compendium, page(s) 219.