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A goblin is behind a rock that gives it half cover. It shoots its crossbow at an adventurer in the open, and the adventurer returns fire with a bow. For the sake of argument, the goblin, the rock, and the adventurer are all in a line on the grid.

Alternatively, the goblin is in a guard tower with a low wall that gives half cover, with the same scenario.

In both cases, who has cover from who?

Obviously, the goblin gets +2 AC against the adventurer's attack, because it's in cover. But does the adventurer have +2 AC against the goblin's attack, because there's half-cover between them?

One argument for the bidirectionality of cover is that it works with full cover; if the rock was big enough, neither combatant would see the other, and thus have full cover from each other. However, it seems weird and unintuitive that attacking from a protected position would give the attacker a penalty.

If the examples I gave don't result in an advantage for the goblin, is there a scenario that does?

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How hard is for you to hit the target is how well you see it which depends on the angle of visibility. See the pic below, the left 'person' sees the other one just as well as if there was no stone and can attack normally, while the one on the right has a limited view and has to contest a higher AC. In this case it is a side view (shooting above the stone) but it works the same way if it were a top view (say, shooting from behind a tree).

That's essentially why arrowslits work.

Cover

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    \$\begingroup\$ I think it's worth saying that the picture is a side view, not a bird's eye view, which most of us are probably used to thinking about in D&D - what, with all the maps! \$\endgroup\$ – Clearly Toughpick Jan 19 '17 at 10:24
  • \$\begingroup\$ @ClearlyToughpick in fact, it can be either, like shooting from behind a tree, but I will clarify it \$\endgroup\$ – black_fm Jan 19 '17 at 10:40
  • \$\begingroup\$ Good point. That raises the question about handed-ness: if the goblin on the left is right-handed, does it take a penalty to make the shot? On second thoughts, don't go there! \$\endgroup\$ – Clearly Toughpick Jan 19 '17 at 10:44
  • \$\begingroup\$ This is a very good answer, but it isn't about angle of visibility, it is about the amount of the target (in resolved angle from the point of the attacker) that is exposed to fire. I concur that if once can't be seen, one can't be engaged, but this is about cover (which is an AC bonus), not about hiding. The angular difference you show above requires the shooter on the left to be ~50% as accurate (in angle space) as the shooter on the right to hit the same sized target. \$\endgroup\$ – tillmas Jan 19 '17 at 12:50
  • \$\begingroup\$ So this makes sense from a real world perspective, but could you add support for this from the rules? \$\endgroup\$ – Icyfire Jan 19 '17 at 16:23
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Partial cover can be bidirectional, but not always.

It depends on how much of the target's body is obscured from the attacker, and not the other way around. In the first two cases you described, the goblin has cover, and the adventurer does not. The PHB defines half-cover in the following way:

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body.

The adventurer's target is the goblin's body, which is half blocked by the obstacle (such as a low wall or boulder). Therefore the goblin has half-cover from the adventurer's attack. This is intuitive.

Whether the cover is bidirectional depends on how the goblin is positioned. How much of the adventurer's body can the goblin see?

Suppose the goblin's crossbow is positioned above the top of the obstacle. The adventurer is standing out in the open. If the goblin can see the adventurer's entire body, then the criteria for half-cover does not apply; the adventurer does not get half-cover from the goblin's attack.

Otherwise, if the obstacle prevents the goblin from seeing half of the adventurer's body, then the adventurer gets the benefits of half-cover from the goblin's attack. This would apply in a situation where the goblin and adventurer are on opposite sides and adjacent to the same obstacle, because it blocks half of their body from the other's sight.

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    \$\begingroup\$ Your last paragraph is a very good point. I think it would be improved with an example (such as a waist high, wide wall or parapet that the Goblin and Adventurer were on opposite sides of). Far enough apart that ranged attacks were realistic (and not with disadvantage), but close enough that both were in half cover from the wall. \$\endgroup\$ – tillmas Jan 19 '17 at 12:54
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As the other respondents have said, the degree of cover that a creature has depends on how much of it an attacker can see and thus target. Cover is always relative to a particular attacker.

There are rules for determining cover when using miniatures on a grid, in the DMG on p251. But they are a bit fiddly to use and don't always yield satisfactory results in my opinion.

When it comes to working out who benefits from cover, a useful shorthand is to consider which of the two creatures is closer to the cover: whoever is closer benefits from the cover while the other does not; if they are equally close, they benefit equally. This is a hangover from earlier editions of the game, but useful enough to retain as a rule of thumb.

Finally, you ask what scenarios give advantage. I think you may be conflating being behind cover with being hidden. If you are hidden from your target when you attack it, your first attack has advantage. Being behind cover can help with hiding and staying hidden, but does not negate the need to hide.

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    \$\begingroup\$ I like this answer, though I have to clarify that I said "an advantage," not the mechanical concept of advantage from 5e. It is a bit confusing, though... \$\endgroup\$ – Icyfire Jan 19 '17 at 19:31

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