What does Blocks Line of Sight really mean?

[Caliber]

So last night our party's Wizard dropped down a Wall of Fog, eliciting the question: What does "blocks Line of Sight" really mean? I remember a pretty hefty discussion of this on these boards sometime ago, but my search-fu is weak.

Essentially, this things seem clear to me (feel free to prove me wrong though!): two creatures separated by a Line of Sight blocking effect cannot see each other.

The fuzzy areas to me are:

A) Can a creature in a square that blocks Line of Sight be seen from outside?
B) Can a creature in a square that blocks Line of Sight see outside?
C) Does positioning matter? Maybe they can see adjacent but not further? Does that work vice versa? (ie, can adjacent creatures see in?)
D) What if both creatures are in the Line of Sight blocking effect? Does that change anything?
E) How do the above rules interact with already existing Concealment rules?

As a tangential note, do you need LoS to Sustain an effect?

Super mondo thanks (and a promise of free XP!) to anyone who helps me and my group sort this mess out.

 

[Ryujin]

A & B depend on what the power being used says. Some effects make the effected square create concealment within them, but block line of sight beyond them. Some effects allow the caster to see normally, so line of sight isn't blocked for him. It's situational.

C) If you can draw a line from a corner of the origin square to a corner of the target square, without passing through the effect that is blocking line of sight, then the target has concealment rather than line of sight actually being blocked. If the effect that would ordinarily block line of sight is solid in this situation, then the target has cover rather than concealment.

D) Can't say that I've read anything regarding sustaining of effects. I would say that line of effect would be needed, but not necessarily line of sight. Perhaps someone else has a credible source?

E) see above.

 

[AbdulAlhazred]

A) Can a creature in a square that blocks Line of Sight be seen from outside?

Technically the creature would have concealment, assuming the attacker can see 2 corners of one square in the target's space.

B) Can a creature in a square that blocks Line of Sight see outside?

The rules don't address this issue adequately, but by the simplest interpretation of RAW the creature inside has no restrictions on its vision.

C) Does positioning matter? Maybe they can see adjacent but not further? Does that work vice versa? (ie, can adjacent creatures see in?)

The same rule applies. If the creatures can see at least one corner of each other's square then they can attack each other.

D) What if both creatures are in the Line of Sight blocking effect? Does that change anything?

Nope.

E) How do the above rules interact with already existing Concealment rules?

They don't really. Concealment and LoS don't really interact except that something you cannot see you have a -5 to hit against.

As a tangential note, do you need LoS to Sustain an effect?

I do not believe so. Just line of effect.

The upshot of all this is that a creature in say a Stinking Cloud would have concealment from any attack, assuming it is at the edge of the cloud and the attacker can see that edge. If the creature attacks, it suffers no penalty since it can draw its LoS from any corner of its square and it will use one which is at the edge of the cloud.