Adaptable Flanker( Player's Handbook II, p. 71)
[Fighter Bonus Feat, General]
When you and an ally team up against a foe, you know how to maximize the threat your ally poses to ruin your target's defenses.
As a swift action, you designate a single opponent as the target of this feat. When you are adjacent to the chosen target, you can choose to count as occupying any other square you threaten for purposes of determining fl anking bonuses for you and your allies. You also occupy your current square for fl anking an opponent.
A fighter can select Adaptable Flanker as one of his fighter bonus feats.