DnD Flanking Rules (Optional and Core)

Flanking is a positional combat mechanic in Dungeons & Dragons 5th Edition that grants attackers a tactical advantage when they surround a target from opposite sides. The rule exists in two distinct forms: a core optional rule published in the Dungeon Master's Guide and informal table variants that groups adopt independently. Understanding which version applies at a given table — and how it interacts with the broader combat framework — determines its strategic weight in play.

Definition and scope

Flanking, as defined in the Dungeons & Dragons 5th Edition Dungeon Master's Guide (published by Wizards of the Coast), is an optional rule found in the "Optional Rules" chapter rather than in the core combat chapter. This classification is significant: flanking is not a default mechanic of 5th Edition. A Dungeon Master must explicitly adopt it for it to apply at the table.

The official optional rule defines flanking as occurring when 2 creatures — each hostile to the same target — occupy spaces on directly opposite sides of that target on the battle grid. When this condition is met, both attackers gain advantage on attack rolls against the flanked creature. Advantage in 5th Edition means rolling 2d20 and taking the higher result, a statistically meaningful benefit that increases the probability of hitting by approximately 5 percentage points against an AC 15 target.

The rule is grid-dependent. It requires a physical or digital battle map divided into 5-foot squares, placing it firmly within the tactical miniatures subset of the game. Tables playing in the "theater of the mind" style — without a grid — have no standard mechanism for determining flanking positions, making the rule inapplicable in that context.

For broader mechanical context, the full conceptual overview of how D&D works covers the action economy and spatial rules that flanking interacts with.

How it works

The official optional flanking rule operates through the following structured conditions:

1. Two attackers required. Exactly 2 creatures hostile to the target must participate. A single creature cannot flank alone.
2. Opposite-side positioning. The 2 attackers must occupy squares on directly opposite sides of the target — not diagonal corners, but cardinal opposites (north/south or east/west relative to the target's space).
3. Adjacency required. Both attackers must be adjacent to the target (within melee reach, typically 5 feet for creatures with standard reach).
4. Advantage granted. Both flanking attackers gain advantage on melee attack rolls against the flanked target for that attack.
5. No stacking. Advantage from flanking does not stack with other sources of advantage — the 5th Edition rule (Player's Handbook, Chapter 7) specifies that multiple sources of advantage still result in rolling only 2d20.

The reach extension case introduces nuance: a creature with 10-foot reach (such as a character wielding a pike, covered under D&D weapons rules) can be in a square 10 feet from the target and still qualify for flanking, provided the opposing flanker is positioned directly opposite at the same extended reach.

Creatures that are incapacitated, prone, or otherwise unable to act still count as valid flanking participants if they occupy the correct squares — the rule does not require the flanker to be actively attacking, only to be positioned correctly at the moment of the attack.

Common scenarios

Two melee fighters surrounding a single enemy. The most frequent application: a Fighter and a Rogue position themselves on opposite sides of a Goblin. Both gain advantage. The Rogue's Sneak Attack feature (which independently requires advantage or an adjacent ally) is automatically satisfied by the flanking advantage, making this combination particularly efficient.

Large or Huge creature flanking. A Large creature (occupying a 2×2 square grid space) is flanked when attackers are on opposite sides of its entire footprint — not merely opposite corners of one square. The Dungeon Master's Guide clarifies that the "directly opposite" determination uses the target's full occupied space as the reference, which expands the number of qualifying positions.

Three or more attackers. Only 2 creatures need to satisfy the flanking condition. Additional attackers on the same target do not gain flanking advantage unless they themselves form an opposite pair with another attacker. This rewards deliberate positioning over simple mob tactics.

Flanking vs. the Help action. A player choosing the Help action to grant advantage to an ally's attack does not need to be on an opposite side — the Help action and flanking are separate advantage sources. In cases where a creature cannot easily position for a flank, using Help may be the more accessible option.

Decision boundaries

The core decision for Dungeon Masters is whether to adopt the optional flanking rule at all. The Dungeon Master's Guide explicitly flags that the rule "makes combat more dynamic" but also notes it can incentivize clustering, which may reduce tactical variety. Published guidance from Wizards of the Coast on the D&D Beyond rules compendium reflects the base game's preference for theater-of-the-mind flexibility over grid dependency.

Optional rule vs. table variant. Groups sometimes adopt a modified flanking rule — for example, granting only a +2 bonus rather than full advantage, or requiring the flankers to spend movement to establish position each round. These table variants sit outside the published optional rule and function as house rules. They are not supported by official errata or Adventurers League organized play documentation (see D&D Adventurers League), which uses the standard rules set without the flanking optional rule.

Interaction with the Dungeon Master's authority. Because flanking is optional, the Dungeon Master determines whether it applies to all creatures equally, including monsters. A Dungeon Master who enables flanking for players but not for monsters is implementing a house rule, not the published optional mechanic. Symmetrical application is the published standard.

For tables concerned about the advantage economy becoming too accessible, the optional rules reference on this site covers additional variant mechanics that can be paired with or substituted for flanking.

The relationship between flanking and opportunity attacks creates a secondary tactical layer: moving out of a flanking position to reposition triggers opportunity attacks, meaning that maintaining a flank carries a defensive cost that limits the mechanic's dominance in extended combat.

References

• Dungeons & Dragons 5th Edition Dungeon Master's Guide — Wizards of the Coast
• D&D Basic Rules (Free Official Reference) — Wizards of the Coast / D&D Beyond
• D&D Free Rules & Rules Glossary — D&D Beyond
• D&D Adventurers League — Organized Play Documentation
• D&D Player's Handbook 5th Edition — Wizards of the Coast