DnD Flanking Rules (Optional and Core)

Flanking is one of Dungeons & Dragons 5th edition's most debated tactical mechanics — partly because the core rules handle it differently than most players expect, and partly because the optional version turns out to be quite powerful once the math gets examined. This page covers both the default rules as written and the optional flanking variant from the Dungeon Master's Guide, how each works at the table, and where Dungeon Masters draw the line on which version to use.

Definition and scope

In 5th edition D&D, flanking does not exist as a standard rule. That surprises players who came from 3rd edition or Pathfinder, where flanking was baked into the base system and granted a +2 bonus to attack rolls against flanked targets. The 5e designers made a deliberate choice to strip it out of the core rules, leaving it as an optional variant.

The optional flanking rule appears in the Dungeon Master's Guide (p. 251) under "Optional Rule: Flanking." When active, two creatures that occupy opposite sides of an enemy — specifically, on sides of a square that are directly across from each other — grant each other advantage on melee attack rolls against that enemy.

Advantage in 5e means rolling two d20s and taking the higher result. The statistical impact of advantage is roughly equivalent to a +3.5 to +5 bonus on attack rolls depending on the target's Armor Class, which makes flanking considerably stronger than the +2 bonus it replaced from prior editions.

The key dimensions and scopes of DnD are worth keeping in mind here: flanking interacts with grid-based play specifically. In theater-of-the-mind games, the spatial geometry required to adjudicate flanking simply doesn't exist in any reliable way, which is one reason the rule stayed optional.

How it works

When the optional rule is in use, the positioning requirement is specific:

1. Two creatures must be on directly opposite sides of the target — not diagonal corners, but across from each other along a shared edge of the target's space.

The geometry becomes more interesting with Large or larger creatures. A Large creature occupies a 2×2 square, so a Medium attacker flanking it must be on the directly opposite side of that footprint — diagonally adjacent corners don't qualify. The DMG is explicit that flanking requires creatures to be on "opposite sides," which on a standard grid means directly across, not diagonally across.

Common scenarios

Two melee fighters sandwiching one enemy is the textbook case — a Fighter and a Rogue positioning on opposite sides of a single target so the Rogue can reliably land Sneak Attack. Since Sneak Attack already requires advantage or an ally adjacent to the target, flanking via the optional rule is a significant upgrade for Rogues in grid-based games.

A reach weapon attacker flanking from range is less intuitive. A Polearm Master Fighter with a halberd (reach 10 feet) standing two squares away from an enemy can still flank with an ally on the opposite side, provided the geometry checks out. The DMG doesn't restrict flanking to 5-foot reach weapons, so reach weapons qualify.

Flanking in a doorway creates interesting problems. If the enemy is standing in a doorway and only one side of their space is accessible, the second flanker may not be able to reach the geometrically opposite square at all. The DnD frequently asked questions page addresses edge cases like terrain obstruction and mounted combat, where the rules leave gaps that Dungeon Masters fill with rulings.

The default (no optional rule) approach uses a different mechanic entirely: the Player's Handbook grants advantage on attacks against a prone or incapacitated target, and the Help action lets one creature grant advantage to an adjacent ally's attack. This produces a similar tactical outcome — two people coordinating to land a blow — without the positional geometry requirement.

Decision boundaries

Whether to use the optional flanking rule is a genuine design decision, not just a preference. The Dungeon Master's Guide itself flags that the optional rule "can incentivize players to move into flanking position rather than use other tactical options," and that's an understatement in practice.

The core tension is this:

• With optional flanking active, melee-heavy parties gain near-constant advantage in multi-enemy fights just by managing positioning. Paladins with Divine Smite, Rogues with Sneak Attack, and Fighters with Action Surge all become substantially more powerful in ways that compress the action economy tightly.
• Without optional flanking, advantage is a limited resource gated behind class features, spells, Help actions, and enemy conditions. It remains tactically meaningful rather than routine.

Most organized play formats — including the Adventurers League, which follows the rules as written — do not use the optional flanking variant. Home tables that run theater-of-the-mind play almost never use it by practical necessity. Grid-based home games split roughly evenly in practice, with DMs who want grittier, lower-advantage combat tending to skip it.

The Help action as the default flanking substitute rewards players for understanding the full DnD rule system rather than simply tracking positional geometry. It requires an action — a real cost — rather than a free bonus from footwork. That cost is intentional, and it's the design difference the optional rule quietly erases.