Must you be able to see your opponents to be flanked?

Question

Neither the entry on flanking from the SRD nor the Player's Handbook entry (p 153) nor the Rules Compendium (pp 56-57) mention anything about being able to see the flanker or being aware of the threat. However, this Rules of the Game Article, All About Sneak Attacks (part three) mentions

You get a flanking bonus from any ally your foe can see (and who is in the correct position to flank). If your foe can't see you, you don't provide a flanking bonus to any ally.

Is there a source for this or is this just an interpretation? (I am aware of the controversy around the RotG articles.)

I was reading Detecting a Held Charge Touch Attack which brought up the topic about how a spell caster threatens when holding the charge. It appears from most of the answers there's some question as to whether it can be detected at all, much less by other means. This then brings up the a related question, If your opponent can't perceive your threat, can you still provide flanking bonuses to allies in a flanking position against that opponent?

If flanking bonuses come from dividing the target's attention, it seems necessary for the target to be aware, but is this ever specifically mentioned in the rules?

It may be simpler to ignore this requirement mentioned in the article, but then it does create the odd situation that within the secondary reality of the game, something an opponent is not aware of has a significant impact on their defense.

I am interested in what the rules say, any indication of rules-as-intended, and recommendations on what works best at the game table.

Answer 1

Note the difference between vision and awareness.

This is extrapolating from RAW but let's take some things into account :

• Creatures with all-around vision can't be flanked.
• Flanking someone is dividing his attention on two opposite fronts.

A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Spot check. The observer gains a hunch that “something’s there” but can’t see it or target it accurately with an attack.

Now imagine you're under the effect of greater invisibility, and your visible buddy is on the other side of the enemy.

If you haven't attacked yet, and the enemy failed his Spot check (or Listen against your Move Silently), not only he can't see you, but he's unaware of the fact that you're there. In this case, he's just turning his back at you, he's not flanked (even though per RAW he should be, and in 3.5/pf there's no notion of orientation).

When you've attacked him, or if he somehow knows something's up, his attention might be divided between fighting your buddy and finding you, in this case he'll be flanked.

Note that this changes nothing for you : if you were alone and he were unaware of your presence you'd get +2 to hit and he'd be denied his Dex to his AC ; and when you flank you get +2 to hit and he's still denied his Dex since you're invisible.

The flanking only helps your buddy in this condition.

Why he should be flanked per RAW

You threaten all squares into which you can make a melee attack, even when it is not your action. Generally, that means everything in all squares adjacent to your space (including diagonally)

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When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by a character or creature friendly to you on the opponent’s opposite border or opposite corner.

Threatening a square is not simply standing there, especially if you're engaged in melee combat. It's not two people standing still and swinging a sword at each other every 6 seconds. There is a fight going on, and this is why you get a flanking bonus.

Conclusion

If you're engaged in combat, and you're fighting on two fronts at once, you're flanked.

If you're invisible and your buddy is on the other side of the enemy, I'd say he'd be flanked only once he's noticed you, be it because you attacked him or because he's heard you coming on behind him.

Thing is, RAW assume you're engaged in combat once you enter an enemy's threat range, or an enemy enters yours (which is logical, you wouldn't stand there and get your ass kicked). But in the case of an enemy who's unaware of your presence, you can choose to engage or not, and therefore whether he's flanked or not.

Answer 2

Disclaimer: This might be just my interpretation on RAI. This is also the way I handle it in my games. YMMV.

Why does flanking work?

Because the defender has her attention divided. She has to be aware of two (or more) attackers. This gives the attackers an advantage since the defender doesn't have eyes on her back.

What happens if I'm the sole attacker and I'm invisible?

The rules are pretty straightforward. I get +2 and the defender is denied the Dex bonus. She is unaware that she has to defend against me.

So... What if I'm invisible and my buddy is engaged attacking?

The defender thinks she's facing a single opponent. Her full attention is on my buddy and she's unaware that I'm behind her back. My buddy doesn't get any bonuses, but I do. Unfortunately, I get the same bonuses as if I were the sole (invisible) attacker.

Answer 3

RAW: Visibility is not mentioned anywhere in the flanking rules, and so has no effect.

...Perhaps every character is assumed to have some level of "Spidey Senses" and is at least unconsciously aware of the opponent.

But...

From the related question is this quote:

Only a creature or character that threatens the defender can help an attacker get a flanking bonus.

(from the d20SRD entry on Flanking)

If the target is unaware, and unable to defend itself properly, because the enemy is invisible (thus causing denial-of-dex, etc), this implies that the target is not threatened by the invisible opponent.

But that's arguing the English definition vs the D&D condition.

There is room for flexibility here, but this would require a DM ruling.