Flanking and Cover Rules
Flanking and cover are two of the most tactically consequential mechanics in Dungeons & Dragons 5th Edition combat — one rewards aggressive positioning, the other punishes it. Together they shape how players and Dungeon Masters think about the grid, creature placement, and the difference between a clean attack roll and one made through a forest of sword arms and shield edges. This page covers both mechanics in full: what they mean by the rules, how they function at the table, where they diverge, and the judgment calls that come up most often.
Definition and scope
Cover and flanking are positional conditions that modify the chance of landing a hit — but they operate on opposite ends of the encounter. Cover is a core rule printed in the Player's Handbook (Chapter 9, "Combat"), granting bonus Armor Class or Dexterity saving throw bonuses to any creature with an obstacle between itself and an attacker. Flanking, by contrast, is an optional rule found in the Dungeon Master's Guide (Chapter 8), meaning it is only active at a table that explicitly adopts it.
That distinction matters enormously in practice. A new player asking about how D&D rules work will find cover reliably present at every table running 5e; flanking may simply not exist at theirs. Knowing which category a mechanic falls into is part of the literacy that makes combat legible — and it's one of the key dimensions of D&D's rules structure worth understanding early.
How it works
Cover operates in three tiers:
- Half Cover — +2 bonus to AC and Dexterity saving throws. Granted when an obstacle covers roughly half the target's body: a low wall, a large piece of furniture, a creature standing in the way.
- Three-Quarters Cover — +5 bonus to AC and Dexterity saving throws. Granted when the obstacle covers about three-quarters of the target: arrow slits, a portcullis, a dense thicket.
- Total Cover — The target cannot be directly targeted by attacks or spells at all. The attacker has no line of sight or line of effect.
The cover bonus stacks with a creature's base AC, which means a fighter with AC 18 behind a low wall effectively has AC 20 against ranged attacks. Against a wizard with a +6 attack modifier, that shifts the probability of a hit from roughly 45% to 35% — a meaningful swing over a full combat encounter.
Flanking, when in use, grants advantage on melee attack rolls to any attacker whose ally occupies the directly opposite side of the target on a grid. Both creatures must be within 5 feet of the target, and the target must be between them in a straight line (horizontally, vertically, or diagonally across a square grid). Large or larger creatures occupy multiple squares, which can extend flanking opportunities — or complicate them.
The gap between these two mechanics is substantial. Cover adjusts a flat numerical bonus; flanking grants advantage, which statistically raises the expected value of a roll by roughly 3 to 5 points depending on the base modifier involved. At many tables, the optional flanking rule is considered too powerful because it trivializes the 5e action economy — two melee fighters working in tandem gain advantage almost automatically in any close-quarters fight.
Common scenarios
Doorway defenders. A creature standing in a doorway fighting off attackers in a hallway typically enjoys three-quarters cover (+5 AC) against any attacker who cannot reach it from the side. Archers down the hall are largely wasting ammunition.
Crowded melees. When 4 or more combatants cluster around a single target on a 5-foot grid, flanking opportunities appear almost by accident. This is the scenario where DMs who run flanking-optional tables most frequently feel the rule's weight — every monster surrounded by two players effectively fights at disadvantage on its own saving throws against being hit, which compounds quickly.
Partial cover from a large creature. If a Hill Giant (a Large creature) stands between an archer and the intended target, the target has half cover. Giants being used as impromptu meat shields is less common in actual play than it should be, and is one of those scenarios that rewards players paying attention to positioning.
Total cover abuse. A creature that ducks entirely behind a wall on its turn cannot be targeted — but it also cannot attack without losing that protection. The interplay between movement, the Attack action, and total cover is a common source of table questions, and the Player's Handbook resolves it cleanly: the cover must exist at the moment of the attack.
Decision boundaries
Three judgment calls come up reliably at the table and deserve clear answers:
Does cover apply against area-of-effect spells? Half and three-quarters cover grant their Dexterity saving throw bonus against AoE effects — but only if the effect allows a Dexterity save. A fireball centered on a creature's square bypasses cover entirely for that creature.
Can a creature provide cover for itself? No. A creature cannot benefit from its own position in space to grant itself cover. Cover is always relative to an external obstacle or creature.
Flanking vs. the Sentinel / Polearm Master combination. Because flanking requires both attackers to be within 5 feet, characters using reach weapons (polearms) technically cannot flank — the optional rule in the DMG specifies melee attacks at 5 feet. This is one of the sharper edges of the optional rule and worth house-ruling explicitly before it derails a session.
For a broader look at how these kinds of judgment calls get resolved at different tables, the D&D frequently asked questions page covers common disputes in detail. And for players new enough to still be finding their footing with the combat system, getting oriented with D&D is a reasonable place to start before stepping into grid tactics.