Flanking and Cover Rules

Flanking and cover are two tactical positioning mechanics in Dungeons & Dragons 5th Edition that influence attack rolls and defensive resilience during combat encounters. Flanking grants offensive advantages to attackers surrounding a target, while cover reduces the likelihood of a successful hit against a defended creature. Both mechanics interact with the core combat rules framework and shape how positioning decisions translate into numerical outcomes on the battlefield.

Definition and scope

Cover is a core 5th Edition rule defined in the Player's Handbook (Chapter 9) that imposes penalties on attack rolls and Dexterity saving throws targeting a creature that has an obstacle or another creature between itself and the attacker. The Player's Handbook identifies 3 distinct cover tiers: half cover (+2 bonus to AC and Dexterity saves), three-quarters cover (+5 bonus), and total cover (cannot be targeted directly).

Flanking is classified as an optional rule in the Dungeon Master's Guide (Chapter 8, "Optional Rule: Flanking"), not a default mechanic. Under this optional rule, when 2 or more creatures occupy positions on directly opposite sides of a target — each within 5 feet — those attackers gain advantage on melee attack rolls against that target. Because flanking is optional, Dungeon Masters must elect to activate it before play begins. This distinction separates it from cover, which is active in all standard games. The optional and variant rules reference covers the broader category of elective mechanics from which flanking is drawn.

How it works

Cover mechanics — structured breakdown:

  1. Half cover applies when at least half of the target's body is obscured by an obstacle (a low wall, a large piece of furniture, another creature). The target gains +2 to AC and Dexterity saving throws.
  2. Three-quarters cover applies when approximately three-quarters of the target's body is obscured (arrow slits, a portcullis, dense foliage). The target gains +5 to AC and Dexterity saving throws.
  3. Total cover applies when a creature is completely concealed behind an obstacle and cannot be targeted at all by attacks or most spells requiring line of sight.

Cover bonuses do not stack. A creature benefits only from the highest applicable tier. The Dungeon Master determines cover by assessing the geometry of the battlefield, typically by drawing a line from any corner of the attacker's space to any corner of the target's space — if those lines pass through an obstacle, cover applies.

Flanking mechanics:

Under the optional flanking rule, 2 creatures must be on directly opposite sides of the target — defined as occupying spaces that form a straight line passing through the target's space — and each must be within melee reach (5 feet for most creatures, 10 feet for those with reach weapons). Both qualifying attackers gain advantage on attack rolls, meaning each rolls 2d20 and takes the higher result. This interacts directly with attack rolls and armor class calculations.

Common scenarios

Doorway defense: A creature standing in a doorway typically benefits from three-quarters cover (+5 AC) against ranged attackers outside the room, as the door frame obscures most of the body. This is one of the most frequently encountered cover adjudications in dungeon-based play.

Cavalry engagements: In open terrain, where obstacles are absent, cover is rarely available. Mounted combat rules introduce additional positioning complexity, but cover adjudication remains the same geometric assessment.

Flanking with reach weapons: A creature wielding a weapon with 10-foot reach satisfies the flanking adjacency condition from 10 feet rather than 5 feet. This means a Large creature or a fighter using a pike can establish flanking geometry from a greater distance, altering the tactical calculus significantly.

Total cover misuse: Total cover prevents a creature from being the target of an attack or a spell that requires a target. Spells with area effects — such as fireball — can still damage creatures behind total cover if the area of effect includes their space, because those spells do not require the caster to target the covered creature directly.

Decision boundaries

The most operationally significant distinction is between flanking as a default rule versus an optional rule. Treating flanking as always-on generates advantage and disadvantage at a rate that can overshadow other mechanical features — particularly those of Rogues, who rely on the distinct Sneak Attack trigger involving allied adjacency. The Dungeon Master's Guide explicitly flags this imbalance as justification for the optional classification.

Half cover (+2) vs. three-quarters cover (+5) represents a non-linear jump. A +5 bonus to AC reduces incoming hit probability substantially more than a +2 bonus — against a typical attack roll modifier of +6 to +8 at mid-tier play, the +5 bonus can reduce hit chance by 25 percentage points or more depending on the attacker's total bonus.

Cover applies to Dexterity saving throws but not to other saving throw types. A creature sheltering behind a half-wall still makes a Constitution saving throw against a hold person spell with no bonus. This boundary is a frequent source of rules questions and is documented in the official Player's Handbook Chapter 9 text.

For Dungeon Masters structuring encounters, the encounter building rules framework treats cover as an environmental modifier that can reduce monster effective hit rates, making encounters feel harder even at the same Challenge Rating. Terrain with consistent three-quarters cover opportunities can meaningfully extend player survivability without adjusting CR. The broader conceptual architecture of how encounter difficulty and environmental rules interact is addressed in the how-recreation-works-conceptual-overview framework for tabletop play.

The full rules reference index at dndrules.com organizes cover and flanking alongside the complete combat and positioning ruleset.

References

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