This rule is no longer variant, it is assumed active by default.

If you regularly use miniatures, fFlanking gives combatants a simple way to gain a bonus advantage on attack rolls against a common enemy.

A creature can’t flank an enemy that it can’t see. A creature also can’t flank while it is incapacitated. A Large or larger creature is flanking as long as at least one square or hex of its space qualifies for flanking.

Flanking on Squares

• When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy’s space, they flank that enemy, and each of them has a +2 bonus to hit advantage on melee attack rolls against that enemy.
• 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.

Flanking on Hexes

• When a creature and at least one of its allies are adjacent to an enemy and on opposite sides of the enemy’s space, they flank that enemy, and each of them has a +2 bonus to hit advantage on melee attack rolls against that enemy. 
• On hexes, count around the enemy from one creature to its ally. Against a Medium or smaller creature, the allies flank if there are 2 hexes between them. Against a Large creature, the allies flank if there are 4 hexes between them. Against a Huge creature, they must have 5 hexes between them. Against a Gargantuan creature, they must have at least 6 hexes between them.

Flanking in the theater of the mind

• In theater of the mind, a creature and its allies qualify for flanking if it and at least one ally are both within range for their melee attacks against the creature they wish to flank.