Flanked

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.

Additionally, a figure can be flanked by a single enemy combatant by being just outside its standard visual range and not directly behind it. This flanking provides a +1 attack.​

​Rear or Flank Attacks
If a creature is able to position itself for a rear or flank attack, it gains a significant advantage. Flank attacks get a +1 bonus to hit, and rear attacks a +2 bonus. A thief attacking from the rear can declare a backstab and gain a +4 bonus to his attack roll. In addition to the attacker advantages, the defender's Shield Spell doesn't help against flank attacks on his unshielded side nor on any rear attacks. Last but not least, the defender's Dexterity adjustment doesn't count against rear attacks, since the defender can't see the attack coming and attempt to dodge it.​