D&D General "that you can see", "line of sight", glass, mirrors, ~clairvoyance, blindsight, and anything else.

[Greenstone.Walker]

When you're casting Hold Person at someone you can see on the other side of a window, what is physically moving from you to the target for the glass to stop?

Or if looked at yet another way, you're not summoning the magic into you-the-caster here, you're summoning it to manifest and do something over there.

Same as with a Lightning Bolt - there's nothing physically travelling between me-the-caster and where I start the bolt, and so I should be able to start it at a point I can see through a window.

The rule about a line of effect implies that there is something physically travelling from caster to target.

The rules don't say exactly what it is, just that there is something making a connection (using Crafword's phrasing) from the spellcaster to their target, something which total cover blocks.

 

[Paul Farquhar]

Try walking through standard commercial glass door and tell me again how it's not something that hinders you.

It hinders me. But it does not hinder a bullet, or a ray of light.

If you stretch the definition enough, solid walls aren't obstructions because they don't stop radar.

Correct, a wall is not an obstruction to radar. It is an obstruction to many other things.

The point is "is an obstruction" is not a property off the object. It will obstuct some things but not others. And, since the D&D rules do not list all possible objects and all possible spells to say what they are obstructed by, it's up to the DM to decide.

 

[Paul Farquhar]

3.5e's rules are simple one set of more detailed rules that could work, and have the benefit of having been tested and discussed by others in great detail.

3.5e rules are a bloated legalistic nightmare that produce results that make no narrative sense.

 

[Rya.Reisender]

For me "see" just requires sight, no matter whether it's through glass or in a mirror.

However, all spells require a clear path to the target. That means there can't be total cover to the target. Casting a spell on a creature behind glass will actually cause it to evoke at the glass on your side. Casting a spell towards a creature you see in a mirror, will cause it to evoke at the mirror.

 

[Sorcerers Apprentice]

Every case where a pane of glass fails to be an obstacle to an object passing through, it's because the glass is destroyed in the process, at least partially.

So a reasonable guideline is that glass doesn't provide cover from a spell if that spell could have targeted the glass itself and destroyed it.

 

[Lanefan]

The rule about a line of effect implies that there is something physically travelling from caster to target.

The rules don't say exactly what it is, just that there is something making a connection (using Crafword's phrasing) from the spellcaster to their target, something which total cover blocks.

In that case I would very much expect the rules to at least give me an idea as to what that "it" is (when not already obvious), both as a guide to how in-setting physics are expected to work for this so I can then extrapolate the same logic to other things as and when needed and as an explanation for the players when (not if) they ask me the same questions. That this guideline isn't given is IMO a pretty big miss.

 

[Paul Farquhar]

Every case where a pane of glass fails to be an obstacle to an object passing through, it's because the glass is destroyed in the process, at least partially.

Not if it's light.

And if it's not double glazed*, sound can path through a glass window without breaking it too.

*Technically, it's the immobile air that is an obstacle to the sound.

 

[Oofta]

It hinders me. But it does not hinder a bullet, or a ray of light.

Correct, a wall is not an obstruction to radar. It is an obstruction to many other things.

The point is "is an obstruction" is not a property off the object. It will obstuct some things but not others. And, since the D&D rules do not list all possible objects and all possible spells to say what they are obstructed by, it's up to the DM to decide.

So a brick wall does not block spells because it would not obstruct x-rays or stop a bullet from high caliber sniper rifles?

 

[Paul Farquhar]

So a brick wall does not block spells because it would not obstruct x-rays or stop a bullet from high caliber sniper rifles?

A brick wall may or may obstruct spells, depending on what the spell is. If the spell happens to be equivalent to an X-ray or a high calibre bullet, then the brick wall is not an obstruction to that spell. I can't think of any spells that a DM is likely to rule that way (Edit: Earthquake perhaps), but they have to make the ruling, since the rules do not say.

 

[Oofta]

A brick wall may or may obstruct spells, depending on what the spell is. If the spell happens to be equivalent to an X-ray or a high calibre bullet, then the brick wall is not an obstruction to that spell. I can't think of any spells that a DM is likely to rule that way (Edit: Earthquake perhaps), but they have to make the ruling, since the rules do not say.

I'd allow a lightning bolt to shatter a window, but that's verging on house rule. Glass is still an obstacle, it's just an obstacle that can be easily broken. I've always run it (in the few cases that it happens) that glass will stop things that have no physical effects, as have other DMs on the few times I recall it happening.

For what it's worth, I dug out my old AD&D 2E book and it says

Spells can be cast through narrow openings only if both the caster's vision and the spell energy can be directed simultaneously through the opening. A wizard standing behind an arrow slit can cast through it; sending a fireball through a small peephole is another matter.​

​

Doesn't really matter to anyone playing 5E of course, just pointing out that certain things have always been and likely always will be a bit vague. In 3.5 spells were blocked by "a solid barrier", 4E calls it a "solid obstacle" and of course 5E just calls it an obstacle.