When are you flanking? (easy question)

[Phage]

So for experienced players, this is likely an easy question, but when are you flanking? I tried searching the forums, but could only find more complex flanking questions.

After reading the rules for flanking and looking at the large creature example in the PRD, I came to a disagreement with another player as to when you get flanking.

Legend:

So P=ally, F=Flanking position, M=monster, and - N/A flanking spot.
Diagram:
123
456
789

I would assume a medium monster would follow the below grids:

FFF
-M-
-P-

Corner
FF-
FM-
--P

I was told it would only be like the below

-F-
-M-
-P-

Corner
F--
-M-
--P

For Large:

For a large creature I would assume

FFFF FFF-
FMM- FMM-
-MM- FMM-
--P- ---P

vs.

-FF- FF--
-MM- FMM-
-MM- -MM-
--P- ---P

Because you can use corners when considering which sides the creatures are touching and in the first grid, cell 8 touches both corners of the medium monster, I thought cells 1 and 3 would be considered flanking.

In the PRD's large example, the Fighter and Rogue didn't count for flanking because the Ogre was so large that the Fighter was basically on the side, however, if you were to reduce the Ogre's size to a single square it would look like the flanking line would touch the both sides of the mob including the corner.

Figured this would be a straight forward question, but new to Pathfinder and this differs from other game mechanics with similar rules so thought I would double check. If I'm wrong, I'm wrong, but would like to know for planning purposes.

[Steev42]

Short answer, you are flanking when an invisible line drawn from the center of your square to the center of your co-flankers square crosses through opposite sides of the flanked creature's square.

In your first case, a line drawn from the center of P to the center of UR M would pass through the bottom and right sides, so would not be flanking.

[Tarantula]

If you draw a line from the center of the base from the creatures you are checking for flanking (fighter/rogue in your example) the line must cross through opposite sides of the flanked creature.

Example: Medium creature, has a fighter to the North, rogue to the south. Line goes from fighter, through creatures N side, through creatures S side, to rogue. Creature is flanked, fighter/rogue get bonuses for flanking. (Only works with melee weapons, fyi.)

Example 2: Same creature, Fighter is to its NW, rogue still to the south.
Line goes from fighter, through creatures W side, through creatures S side, to rogue. Not flanked, as W and S are not opposite sides.

[Bobson]

Going through opposite corners is also valid. The flanking rules make for some weird situations. A colossal creature (6 space base) can be flanked by two people each of whom have 6 spaces they can stand in and it'll still be flanking. But if one moves so they're diagonal from the colossal creature, the other is restricted to being on the opposite corner to flank. So there's 36 flanking layouts for the sides (not 6), but only one for the corners.

[AvalonXQ]

Bobson wrote:

Going through opposite corners is also valid. The flanking rules make for some weird situations. A colossal creature (6 space base) can be flanked by two people each of whom have 6 spaces they can stand in and it'll still be flanking. But if one moves so they're diagonal from the colossal creature, the other is restricted to being on the opposite corner to flank. So there's 36 flanking layouts for the sides (not 6), but only one for the corners.

A rule to cure this ridiculousness would be to just allow a "square deviation" up to half the face of the creature (round down) to still allow flank. Medium flankers would be unaffected, but for larger creatures, there would be the same number of squares that you can flank with an ally regardless of how that ally is positioned.

[Phage]

The main confusion was coming from the "(including corners of those borders)" clause since it is ambiguous as to whether or not this would affect what sides are considered touched with the drawn line OR if it just meant repeat on diagonals.

I interpreted it as including the corners with any of the drawn lines because it mentions nothing about diagonals, only what sides the drawn line would be touching. In this case the drawn line from cell 8 to cell 1 would touch the bottom right corner and the top left corner, which is a valid diagonal flanking and cell 8 would also be touching the bottom right corner so it could give cell 3 flanking as well. Because Flanking often represents the monster's field of vision this seemed within reason.

If the community as a whole prescribes to it needing to be directly across from each other for flanking purposes, that's fine and I'll go with that, but the actual wording and diagram seem very misleading. I really don't get why they wouldn't have a straight medium versus medium diagram in the core book since so many monsters are humanoid NPCs being small or medium.

[Maldollen]

Phage wrote:

In this case the drawn line from cell 8 to cell 1 would touch the bottom right corner and the top left corner

Draw the line between the centers of both squares, and the line from cell 1 to cell 8 passes through the wall between 4 and 5 and then through the wall between 5 and 8, and thus is not flanking.