Flanking

One of the simplest ways for two allied creatures to gain combat advantage is for them to take up flanking positions adjacent to an enemy.

Combat Advantage: A creature has combat advantage against any enemy it flanks.

Opposite Sides: To flank an enemy, a creature and at least one of its allies must be adjacent to the enemy and on opposite sides or corners of the enemy’s space. When in doubt about whether two creatures flank an enemy, trace an imaginary line between the centers of the creatures’ spaces. If the line passes through opposite sides or corners of the enemy’s space, the enemy is flanked.

Large, Huge, and Gargantuan Creatures: A Large or larger creature is flanking as long as at least one square of its space qualifies for flanking.

Restrictions: A creature cannot flank an enemy that it can’t see. A creature also cannot flank while it is subject to any effect that prevents it from taking actions. If no line of effect exists between a creature and its enemy, the creature cannot flank the enemy.

Published in Player's Handbook, page(s) 285, Rules Compendium, page(s) 218.