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We have four medium creatures, blue (1), green (2), yellow (3), and red (4), positioned like so:

enter image description here

We want to asses the relative cover between blue and red. The rules for determining cover on a grid state:

To determine whether a target has cover against an attack or other effect on a grid, choose a corner of the attacker’s space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover. If three or four of those lines are blocked but the attack can still reach the target (such as when the target is behind an arrow slit), the target has three-quarters cover.

Following the instructions here, I have this diagram:

enter image description here

This appears to be 3/4 cover: all four lines are blocked, yet the attack should still be able to reach the target since these creatures do not occupy their entire spaces.

But I am not so sure this is 3/4 cover. The general rules for cover state:

If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.

Considering green and yellow as individual sources of cover, we see:

enter image description here

Each only individually provides half cover. Do green and yellow combine to provide 3/4 cover as in the first cover diagram, or do they together still only provide 1/2 cover since degrees of cover do not add together?

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    \$\begingroup\$ Related, but a slightly different situation: How to resolve multiple sources of cover when using a grid? \$\endgroup\$ – Thomas Markov Dec 9 '20 at 18:40
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    \$\begingroup\$ Well asked, the pictures are very helpful for understanding the question! \$\endgroup\$ – kanoo Dec 9 '20 at 20:53
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    \$\begingroup\$ Gentle reminder to everyone to Please not answer in comments. If the question has a misunderstanding about the rules they are asking about, please address those in an answer. If it's already addressed by an answer upvote those and if it's a minor detail you can suggest it as an addition to one of the answers (or give your own, full answer). \$\endgroup\$ – Someone_Evil Dec 10 '20 at 16:56
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Three-quarters cover

As stated above, the rules for determining cover on a grid read:

To determine whether a target has cover against an attack or other effect on a grid, choose a corner of the attacker’s space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover. If three or four of those lines are blocked but the attack can still reach the target (such as when the target is behind an arrow slit), the target has three-quarters cover.

There is an order of operations to this:

  1. Draw lines from a corner of the attacker's space to each corner of a square the target occupies.
  2. Check if any of the lines are blocked by an obstacle.
  3. Determine level of cover based on number of lines blocked.

Specifically, you choose the corner before checking for obstacles, you do not pick which corner you are using to test cover for which obstacle. So you could choose the top-left corner (which only has half cover from creature 3, but has three-quarters cover from creature 2), or you could choose the bottom-right corner (which has half cover from creature 2, but has three-quarters cover from creature 3). Once that decision has been made and cover checked is when the cover rules from the Player's Handbook come into effect:

If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.

In either case, one creature is providing half cover and one creature is providing three-quarters cover, so you would use the most protective degree of cover: three-quarters cover.

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    \$\begingroup\$ While I realize sanity is not a prerequisite for rules interpretations, this does have the advantage of being saner than the alternative. Saying "the upper left is half-cover through 3, and the lower right is half-cover through 2, so it's half-cover overall" implies the attack simultaneously originates from opposite sides of the 5' square, which is clearly nonsensical. This approach is "the attack originates from a point within the square of your choice" which allows optimization, without involving "Wanted"-style curved bullets. \$\endgroup\$ – ShadowRanger Dec 10 '20 at 15:36
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    \$\begingroup\$ @ShadowRanger and there's already a well-defined mechanism for "Wanted"-style curved bullets anyway: the Sharpshooter feat ignores 1/2 and 3/4 cover. \$\endgroup\$ – TylerH Dec 10 '20 at 16:22
  • \$\begingroup\$ @TylerH I like to think that the sharpshooter is just so accurate that the cover is inconsequential. But sure, maybe the attacks are just curving around the obstacles. \$\endgroup\$ – smbailey Dec 10 '20 at 17:56
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    \$\begingroup\$ @ThomasMarkov: It's the only logical basis for defining cover the way they do (sanity & rules need not coincide, but it's nice when they do). The 5' square is an abstraction of your character's position; they're somewhere in the square, and the cover rules seem inclined to do the sane thing: treat their real position as being "the place with the clearest shot" (your character isn't a moron; if shifting a couple feet gives them a clearer shot, it's assumed they'll do so). You only handle corners because it's simple and in 5' land, the best position is always equivalent to at least one corner. \$\endgroup\$ – ShadowRanger Dec 10 '20 at 18:06
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    \$\begingroup\$ @TylerH Sure, and if my player wants to describe it as curving arrows--as long as no mechanics change--I have absolutely no problem with that. \$\endgroup\$ – smbailey Dec 10 '20 at 18:10
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Half cover

If one or two of those lines are blocked by an obstacle (including another creature), the target has half cover.

2 creatures are 2 obstacles, not “an obstacle”; therefore they are multiple sources of cover and each is assessed individually as per your second quote.

Notwithstanding, a creature (or anything else) that only gives half cover can never cause more than half cover no matter how many lines it blocks because:

A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

Any number of half cover obstacles cannot give three quarters cover. So if the obstacles can only every cover half of the target, such as a creature behind a similar sized creature, then you cannot “up” the cover by having more of them.

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    \$\begingroup\$ I don't think using a creature as an example of something that can block half of the target's body necessarily means that all creatures can only provide half-cover. Do tiny creatures (say a mouse) and huge creatures (say a dragon) provide the same level of cover? \$\endgroup\$ – smbailey Dec 9 '20 at 22:10
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    \$\begingroup\$ @smb but a small creature like a mouse would not be an obstacle "that blocks at least half [the target's] body." (Unless the target was something similar in size to the blocking mouse). \$\endgroup\$ – Rykara Dec 9 '20 at 23:07
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    \$\begingroup\$ If you're a halfling hiding inside a wardrobe, is that half cover because "large piece of furniture" is on the list of examples? \$\endgroup\$ – Mark Wells Dec 9 '20 at 23:08
  • \$\begingroup\$ @Rykara I agree, but Dale's argument seems to be "a creature is a half cover obstacle by definition", and a mouse is a creature. \$\endgroup\$ – smbailey Dec 9 '20 at 23:14
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    \$\begingroup\$ @jonDraco The grid rules do seem to imply that tiny creatures would provide cover in that case. If I was running it I would personally say (and I think most GMs would agree) that mice and most other tiny creatures are not large enough to be considered as an obstacle for the purposes of determining cover. The grid rules only consider a two-dimensional battlemat, but thinking three-dimensionally it just doesn't make sense, and the general cover rules say a creature has half cover if "an obstacle blocks at least half of its body". I couldn't see 4 mice blocking half of a medium creature's body. \$\endgroup\$ – smbailey Dec 10 '20 at 22:20
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Unless you want to spend time tracing lines at the table, just use the text in the parenthesis to infer.

such as when the target is behind an arrow slit

Apply half cover if something is blocking the path (including one or more creatures), unless you think the cover is equivalent to being behind an arrow slit, then use 3 quarters.

enter image description here

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