Objects block line of effect for most spells and supernatural
abilities, though some of these (such as detect magic) can
penetrate barriers and others (such as teleport) ignore barriers.
Line of effect remains unblocked if there is at least 1 square
foot of open space per 25 square feet of barrier (5-foot-by-5-
foot section). For example, a closed door that is 5 feet wide and
8 feet high and that has no openings other than a keyhole
blocks line of effect. Replace the door with a set of bars or a
fishnet, and line of effect is no longer blocked. A small object,
such as an overturned table, won’t block line of effect unless it
is at least 5 feet high and 5 feet wide. If it’s smaller than that,
there’s enough space around the edges to let magical effects
pass right by . (Such an object can still provide cover, though).
A creature generally doesn’t block line of effect because it
almost always leaves the requisite 1 square foot of open space
per 5-foot-square area it happens to occupy. A lightning bolt
spell or a black dragon’s breath weapon can rip through any
number of such creatures standing one behind the other in a
line, to the limit of the effect’s range. For example, a lightning
bolt spell cast at 10th level can create a line 5 feet wide and 200
feet long. Forty Medium-size creatures could stand single-file
in an area that size, and a bolt with those dimensions would
affect them all. Furthermore, the creatures at the front of such a
line do not provide cover against the effect to the creatures
behind them.
Some creatures, however, can completely block off the
requisite amount of area. For example, a gelatinous cube can
completely fill a corridor that is 10 feet square. If it does so, it
blocks line of effect for all spells. A creature that is
significantly bigger than a spell’s area also can block line of
effect. For example, a spell with a line- or cone-shaped area
could be blocked by a colossal dragon standing near the spell’s
point of origin—sometimes there’s just no way to aim a 5-footwide
stroke of lightning so that it can get past a really big body.