D&D 5e’s optional Flanking rules are probably the most controversial rules I’ve encountered since I started playing catch-up, back in September. I don’t want to bury the lead on this one, so I’ll come straight out and publicly declare, at least for the time being, when using miniatures and tactical maps, I’ve decided to follow Taking20’s lead and adopt a Flanking house rule which simply grants +1 To-Hit for flanking combatants, as opposed to the RAW (below), which grant Advantage.
That’s all you really need to know. Of course, if you want to sound like a complete nerd at your next fantasy fiction convention, by all means, do read on, and then do some Google searches.
Page 251 of the Dungeon Master’s Guide offers the following:
Optional Rule: Flanking
If you regularly use miniatures, flanking gives combatants a simple way to gain 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 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 advantage on 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.
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