Part of an ongoing series of 5e Rules notes.

Like all things 5e, WotC set out to simplify the mechanics of how people were protected out on the battlefield by various objects.

5e set up basically four conditions:

1. no cover / uncovered (the default)
2. half cover
3. three-quarters cover
4. total cover.

The first and last usually get treated separately. It’s the partial covers in the middle that are of most interest here.

It’s difficult to talk about cover separated from a battle-map. Or, rather, if you are just running Theater of the Mind, cover is a matter of the GM asserting it (or agreeing to player assertions about it) by fiat. A lot of the below will depend on working on a square grid (extensible to a hex grid, if one likes; check out the DMG pages referenced below).

How about a drawing and a table?

The key here as to what cover a target has is counting the points on any one of their squares from any of the points in your square to see how many are blocked.

So here are the effects of cover on attacks, based on the rules here. This most often comes into play with Ranged attacks (including Spells), but

Points Blocked	Cover Type	AC and DEX Saves	Examples
1-2	Half	+2	Low wall, large furniture, narrow tree trunk, or a creature* (friend or enemy) directly in front of them
3-4	Three-Quarter	+5	Portcullis, arrow slit, thick tree trunk. Any of the target visible.

^{*A creature at least half as large as the target standing next to them.  But … see my House Rules below.}

Points Blocked: As in the diagram above, on a grid, choose a (most favorable) corner of the attacker’s space. Trace a line from that corner to each of the corners of a square (any one) the target occupies. Based on how many of those points are blocked, you can determine the level of cover.

So if any of the points are blocked, there is at least Half Cover. But also note that, even if the all the corners are blocked (e.g., the target is behind an arrow slit), if you can see any of the target, it’s in Three-Quarter cover.

Total Cover:  A target that cannot at all be seen / is completely concealed cannot be targeted by an attack or spell (though some spells can reach it in an Area of Effect — Fireballs, for example). Total Cover also starts to invoke rules for Hiding and the like.

Sizes of the characters involved can affect this (Small creatures behind larger creatures, etc.).

Multiple Covers provide the most difficult cover level. Arguably shooting an arrow past four people is more difficult than shooting an arrow past one person, but the KISS principle applies. As GM you can rule a cluster of Half Covers equal a Three-Quarters Cover, but the Rules As Written say that it’s still only Half Cover.

Combat and Cover at Corners

Consider the case in the picture — Fighter and Kenku squaring off (so to speak) at an architectural corner. Do the have cover from each other?

It might seem so, especially since the Move rules for grids indicate you can’t move through such a corner (PHB 192):

Corners. Diagonal movement can’t cross the corner of a wall, large tree, or other terrain feature that fills its space.

But for combat purposes, there’s no cover, because the kenku can take his top two corners (or the fighter his right two) and see (allowing for map/grid irregularities) along the wall all the other points of the opponent’s square.

It seems counter-intuitive, but there you are. Similar rulings can be made around doorways (the three squares on the other side of a 5-foot door have no cover from someone standing in the doorway on the other side, treating walls has having no thickness).

House Rule: Proximity to the Obstacle

Proximity to the Obstacle:  Rules as Written say that obstacle are obstacles. My House Rule is a little more nuanced:

The attacker can ignore Half or Three-Quarter Cover if the attacker is closer to the obstacle than the target.

It’s all a matter of perspective. If an ally is right in front of me, I can weave around in my 5-foot square to get a clear shot; if they are right in front of the target, they provide much better cover for that target.

Take three examples that I will, for no particular reason, label as William (W) and Moony (M) dealing with a Goblin (G).

1) W---------->MG
2) WM---------->G
3) W-----M----->G

The normal use case is #1, where Moony is up there whomping on the Goblin, and William is behind, shooting a bow at the Goblin. That’s pretty clear; the Goblin gets Half Cover from Moony against William’s bow shot.

Consider case #2, where Moony was right in front of William. The penalty shouldn’t count here; it’s easy in a 5-foot space for William to shoot past Moony at the Goblin, adjust to shoot over Moony’s shoulder or to one side or the other. Assuming Moony isn’t doing jumping jacks in front of William, and is of a comparable size, that makes sense.

Use case #3 — where Moony is midway between William and the Rat is a bit more dodgy (so to speak). The angle to shoot around Moony is more difficult, though not as difficult as when Moony is right in front of the Rat.

So, what’s the ruling here? 5e would treat all three circumstances as providing cover, but I don’t like that. So I’ll borrow from the 3.5e rules:

Attacker can ignore the cover if he’s closer to the obstacle than his target.

At least as applies to Half and Three-Quarter Cover. In case #1, cover rules apply; in case #2, they do not; in case #3, William would need to take a step forward to fire and ignore the cover.

Some Other Notes

• Note this is one of the few cases were 5e bakes in simple bonuses (vs using Advantage/Disadvantage). Assuming Advantage gives you about a +4 on a roll (it varies), that becomes too crude a measure for this.
• There is a Variant Rule (DMG 272) about the chances of hitting the cover if you miss your target. KISS, man. Also, we’ll assume that people are being particularly careful not hit their allies.
• Note that the Sharpshooter and Spell Sniper feats basically do away with Cover for their user. That’s pretty cool.

Update: OneD&D

In “Unearthed Arcana 2022 – Expert Classes,” the Hide action is allowed when behind Three-Quarters or Total Cover.  The Sharpshooter and Spell Sniper feats ignore Half and Three-Quarters Cover, as in 5e.