An opponent is flanked if the opponent is threatened by two characters or creatures friendly to each other, able to attack the opponent, and located on the opponent’s opposite border or opposite corner. An opponent grants combat advantage to the characters flanking it.
When in doubt about whether two friendly characters flank an opponent in the middle, trace an imaginary line between the two friendly characters’ centers. If the line passes through opposite borders of the opponent’s space (including corners of those borders), then the opponent is flanked.[PH:285]