Line of Fire

A figure's line of fire is considered to run from the center of its square to the center of the target's square. If the line of fire passes through any part of a square containing an obstacle, the target is considered to be covered or concealed to some extent. The DM can decide whether the target is 25%, 50%, 75%, or 90% screened, or he can roll randomly if it is not clear. If the line of fire passes through a square containing a figure engaged in melee, the archer must use the rules for firing into a melee to determine where his shot goes.

Note that intelligent creatures who are facing the archer will usually use the screening object for its maximum effect, while animals and monsters that aren't intelligent (Int 1 or less) will rarely try to screen themselves against an archer's fire.

If there's any question about which squares the line of fire passes through, you can use a string, a ruler, or the edge of a card to check the missile's path.