Is 'cover' simply about being out of sight? What IS cover?

Question

The thread 'What's my most efficient use of spell slots to maximize my AC' sparked this line of thought:

Is a curtain 'cover', even if enemy spell casters, archers (etc.) know a PC is behind it? For example: in the middle of combat, could a character summon an unseen servant to hold a banner/flag/cape up--in the middle of a room or field--and gain 'cover'? Is simply being hidden so that enemies could no longer actually lay eyes on the PC, what 'cover' involves?

Or does the object giving cover have to be substantial enough to stop arrows, etc.? Would a barrier give 'cover' vs. magic missiles simply by blocking visual sight of the target, but not give (as much?) cover if arrows/javelins/etc. are being launched?

Could the PC cast (some) spells from behind the curtain if there was a small eye hole to peek through? To what extent would that eye hole undo the 'cover' provided by the curtain?

My original focus was on the unseen servant holding up the curtain and other threads (such as 'Unseen servant use in combat') touch on this, but don't directly answer my questions, as far as I've seen. But from comments/etc. I'm refocusing on the question of what 'cover' means, with the unseen servant in the example to give a specific example around which to focus discussion and show a practical application of the idea.

About 'cover':

The 5e PHB p.196 says,

A target has half cover if an obstacle blocks at least half its body.

A target with total cover can't be targeted directly by an attack or spell, although some spells reach a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

More was said there on the topic, but I didn't see a clear statement against the idea that a PC has cover simply by being hidden from view.

...On the other hand, the objects mentioned that blocked sight of the PC were more substantial than a curtain: tree trunks, low walls, furniture, and other creatures.

My thoughts: an arrow shot at a curtain would pierce it and could wound the person behind it, but doing the latter would be harder because of the curtain. Thus, a curtain = at least some degree of cover.

A magic missile spell depends on the caster being able to see the target (5e PHB p257, emphasis mine):

Each dart hits a creature of your choice that you can see within range.

That seems like a reasonable basis for saying the PC behind the curtain could not be hit with magic missiles.

Other spells might not be as specific and could be figured out on a case by case basis.

As far as whether the eye-hole would allow the PC to cast spells from behind the curtain, from that reading of that spell, I would say it would allow casting magic missiles from behind the curtain.

Answer 1

The definition of cover is a physical barrier between a target and something trying to affect it.

A physical obstruction between you the space a creature may be in means you cannot target that space or anything that may be in it-- the obstruction itself frustrates efforts to operate through it.

This is distinct from a case like an invisible enemy, where you can guess where it might be and target that space with Disadvantage, representing that the target may or may not be where you aimed (plus the normal chance of missing, represented by the d20 roll).

It's a minor point on the main question, but being out of sight is not the same as being hidden. Hiding has specific mechanics in 5e, and simply walking out of view (or even being invisible!) is not sufficient.

Some specific cases of cover:

1. For casting spells, a "clear path" is a hard requirement, and total cover prevents that

You cannot cast a spell on something if there is an obstacle between you and it: if an enemy is standing in the open, you could cast Toll the Dead on it. If that same creature steps behind an opaque curtain, the curtain blocks the path, and so it is no longer a valid target for the spell. The curtain provides total cover, and that's that. The same goes for a glass window: it's an obstacle which blocks the spell's path. Some spells are exceptions, such as Sacred Flame, but they will state their specific properties which override the general rule.

Depending on the spell, you may be able to target the cover itself for secondary effects (like starting a fire, which may then go on to damage the creature behind the cover), but that is highly situational. Additionally, some spells are limited in what kinds of targets are valid, such as Eldritch Blast. The only valid targets of that spell are creatures, so you could not target the curtain itself.

2. For physical attacks, total cover means you can't target the creature behind the cover

This case is similar to the spellcasting case above. If an enemy steps behind an opaque curtain, you can't see them and they cannot be targeted. That doesn't mean that you can't target the cover itself or strike the area behind it (weapons and feats permitting), but that may not be effective in damaging the creature. In that case, I'm thinking of a character using a spear (or other weapon with the Reach property) to stab through a flimsy, but opaque, curtain into the space beyond.

3. Partial cover represents the target being harder to hit due to an obstacle

Half or three-quarters cover indicates that you can see some portion of the target and could target it, but the cover itself interferes with actually hitting the target with an attack. As noted by Mark Wells in comments, the only examples in the PHB of things providing partial cover are solid objects.

If you're an archer, and your target is standing behind a low stone wall, they may have half or three-quarters cover (depending on the heights of the wall and creature both), which your DM will determine. The wall is a physical obstacle and its presence means that your arrow might strike that obstacle rather than your target, even if you "would have" hit the target were the obstacle not there.

This is represented by the target behind partial cover having bonuses to AC and Dexterity saving throws. They're valid targets, but are one way or another harder to hit than if the partial cover were not there.

4. Can I cheese it? Ask your DM

It's a valid tactic to destroy or degrade cover, and it's up to the DM to adjudicate when this has happened. In your hole-in-the-curtain example, I might allow a difficult Perception check. But I would not say that the existence of any hole or gap in the curtain means the creature behind it is visible and therefore a valid target.

An eye hole in a curtain 60 feet away from you definitely does not guarantee that you can see the thing behind it in any meaningful way, especially if playing on a grid (5 square feet is a lot of space to move about in, and you definitely would not have much field of view through such a tiny opening).

On the other hand, if you yourself are hiding behind a curtain and are using the eye hole to see your own targets, that's much more defensible. It's not different than looking out of an arrow slit in a stone wall or over a crenel, really. It would be sensible to restrict your field of view somewhat, or limit some spells (such as a Fire Bolt coming from your hands, and so needing some way to pass through the curtain) but those are not reflected in the mechanics directly.

Regardless, a DM that wants you to have a game any harder than a cakewalk will easily find ways to annihilate or elide such flimsy cover, to prevent it from being a dominant tactic. There are many options for this, particularly area-of-effect spells.

Answer 2

Cover is not just not being able to see someone. Someone behind a curtain, like you used as an example, would be hidden, whereas someone behind a window would be behind cover. If they new the person was behind the curtain, if the curtain was thick enough to block attacks such as arrows, it would be cover, and if it wasn’t, it would not count. Cover is the ability of an object to block attacks. Someone completely behind a rock has full cover, if their legs are sticking out, only half cover.

Spells like fireball are not blocked by cover, because you don’t need to make an attack. Spells like Firebolt, however, would be blocked, as it requires a spell attack. In fact, you can even cast fireball (or other area attack spells) if you can’t see the target. For your question about the eyehole, I’d rule no, unless your spellcasting focus can fit through the hole as well (though I agree that magic missile would be an exception to this).

A better example I think would be a thick wooden door. You open it, cast fireball, then close it and you are behind full cover. Completely workable. You could even have your Unseen Servant open the door for you.

Magic Missile is not technically an attack. If you know there is an invisible creature, you can swing around randomly and hope you hit it. Not with MM. Cover doesn’t apply to Magic Missile. In fact, you could Magic Missile a guy separated from you by a ten foot thick stone wall if you were scrying the other side.

Overall rule: cover only applies to attacks.

Hope this answered all your questions.