Flanking

Flanking. When you and at least one of your allies are adjacent to an enemy, and on opposite sides or corners of the enemy's space, you flank that enemy. Flanking an enemy gives both flankers a +2 bonus to melee attack rolls against that enemy.
Limitations. You can't flank a creature under the following circumstances

• You can't see the enemy
• You are Incapacitated
• You are currently being flanked

Larger Creatures. A large or larger creature is flanking as long as at least one of its squares qualifies for flanking.

When in doubt about whether two creatures flank an enemy on a grid, 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.

If two sets of two creatures flank each other at the same time, the ones who started the flanking first become the flankers. Often, this means that creatures with the higher initiative becomes the flankers. Some circumstances might be up to DM interpretation, such as when two creatures spring a surprise attack in the middle of combat, against already acting creatures.