Line of sight and line of effect are two concepts that can confuse new players and dungeon masters for Dungeons and Dragons 4th Edition. I know that I have personally struggled with these, and it feels like there’s always something new to learn. Below are some basic pointers to help you understand line of sight and line of effect and what they mean for you and your game.
What is line of sight?
Line of sight means exactly what it says: You’re able to see a thing from where you’re standing. What blocks line of sight?
- Solid, opaque objects (walls, doors)
- Complete darkness
- Certain magical effects that specifically say they block line of sight (such as a wizard’s Stinking Cloud)
An interesting case of an object that does NOT block line of sight is a solid, transparent object such as a pane of glass or a transparent crystal. If you can see through it, it doesn’t block line of sight. Also, dim light and fog and similar things might grant some concealment, but they don’t block line of sight. You can still see through them, even if only dimly.
Creatures also don’t block line of sight. It’s assumed that the creature is moving around enough in its square that you can still make out what’s on the far side of it (even though the creature in the way might grant cover if it’s your enemy).
What is line of effect?
Line of effect means that something going from point A to point B won’t get stopped by anything. What stops line of effect?
- Solid objects, whether opaque or not (doors, walls, even glass or crystal)
- Certain magical effects that specifically say they block line of effect (such as spells that generate solid walls)
Things like darkness and blindness and invisibility don’t matter one bit for line of effect – an object would not be impeded at all if it were going through a cloud of magical fog or darkness, so line of effect still exists through them. Creatures also don’t stop line of effect (again, they’re assumed to be moving around in their squares), though they may grant cover.
The canonical example of something that blocks line of sight but not line of effect is darkness. The canonical example of something that blocks line of effect but not line of sight is a pane of clear glass. Keep those examples in mind, and you should be able to figure out what’s what.
In order to make a melee attack against something that’s in range of your melee attack, you must have line of effect to the target but you don’t have to have line of sight. As long as your axe can get there, it doesn’t matter if you can see the target or not – you have line of effect and are allowed to attack. If you can’t see it (you’re blinded, it’s totally dark, etc.) then the target has total concealment from you, which means your attack has a -5 penalty to hit. But you can still make the attack.
As with a melee attack, you only need line of effect to the target with a ranged attack, not line of sight. If you’re shooting an arrow through a cloud of magical darkness at a creature on the far side, the darkness does nothing to stop your arrow. Again, if you can’t see the target it has total concealment – a -5 penalty to the attack roll.
The hooded archer can shoot the goblin through the black cloud of magical darkness with a -5 total concealment penalty; he can't shoot the rat on the far side of the glass wall
With a close attack, you only need line of effect to the target. Your Thunderwave doesn’t care if you can see something or not – it just has to be able to get to it. In addition, concealment doesn’t matter for close attacks, so even if the target is invisible in a completely dark room, your close attack has no penalty to hit it.
Somewhat surprisingly to me, area attacks work pretty much as close attacks do. With an area attack, there are two different things to consider – the line from the caster to the origin square of the burst, and then the lines from the origin square of the burst to the targets that will be hit by it.
In order to put an area burst’s origin in a particular square, the caster only needs line of effect to that square, not line of sight. If the wizard closes her eyes and points, she can still have the magical burst originate exactly where she wants it to. If she’s shooting through an arrow slit , that’s good enough – the magical energy can get through the gap and erupt right where she wants it. She can’t place the magical effect on the opposite side of a pane of glass, though – she does need line of effect to that origin square.
Even if the green wizard is blind and the cloud of gas blocks line of sight, she can still have her Area Burst 2 attack originate in square 1 and hit the fungus creature
As for the burst itself, it works just like a close attack. As long as there is line of effect from the origin square of the burst to the target, the target will get hit. This lets the wizard “shoot around a corner” as well – she can place the burst at the intersection of two hallways, and the burst will shoot down the side passage to hit any creatures within range.
The basics of line of sight and line of effect for attacks are that you always need line of effect and you never need line of sight. However, if you don’t have line of sight, then your melee and ranged attacks will suffer a -5 penalty from total concealment (but your close and area attacks are unaffected).
In a future post I’ll talk about a concept that has some connection to line of sight – being hidden.