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This answer (let's call it A) argues that normally the distance to another plane is infinite.

This answer (call it B) states that you can be affected by fireball damage while in a portable hole hole, which is an extradimensional space. This question about casting spells through planar gates in general was closed with the argument that it is a duplicate, which would mean (i) there is no fundamental difference between a plane and an extradimensional space, and (ii) as you can be reached by the fireball's area of effect, which extends to a distance of 20 feet, the distance to the extradimensional space is not infinite.

How can both (A) and (B) be true at the same time? Do open portals, gates or other interplanar transition points change the distance between planes from infinite (or at least undefined) to 0 at their exact location, and turn a distance that can be measured from there into normal, continuous distances?

For example: You are 10 feet away from a gate, your familiar is 40 feet away from the gate on the other side.

  1. Can you walk 30 feet toward your familiar and then be only 20 feet away from it?
  2. Can a goblin next to you shoot the familiar with a short bow?
  3. Can a goblin shaman next to you target the familiar with fire bolt?
  4. Can you see through the familiar's eyes?
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It's complicated

Mordenkainen, in his second treatise on inter-dimensional dimensions, postulated that....

No, wait, that's not right, because....

The rules don't say

The rules don't define inter-dimensional dimensions. The rules don't define what an extradimensional space is, or a planar gate, or any of those related phenomena, in any detail. They're defined generically, if at all, and then the features that use them define how they work.

But to answer your question

"How can both (A) and (B) be true at the same time?"

1. Magic

Of course extra-dimensional spaces are going to mess with your dimensions. That's why they're "extra" dimensional. And how can "another plane of existence" possibly obey three-dimensional geometry?

2. You're mixing specific and generic

A Portable Hole is relatively well defined, in that we have a definition of a specific object.

The terms "gate" and "portal" are extremely generic, within the rules. If you were talking about a specific spell or magic item, then maybe there would be some answers.

It's perfectly reasonable that one gate or portal would allow you to throw a rock through to the other plane or whatever, but another gate or portal wouldn't.

Planar portals are discussed in the DMG in Planar Travel, under Planar Portals:

“Portal” is a general term for a stationary interplanar connection that links a specific location on one plane to a specific location on another. Some portals function like doorways, appearing as a clear window or a fog-shrouded passage, and interplanar travel is as simple as stepping through the doorway. Other portals are locations — circles of standing stones, soaring towers, sailing ships, or even whole towns — that exist in multiple planes at once or flicker from one plane to another. Some are vortices, joining an Elemental Plane with a very similar location on the Material Plane, such as the heart of a volcano (leading to the Plane of Fire) or the depths of the ocean (to the Plane of Water).

Clearly just from that paragraph, it isn't so much that there are different kinds of portals that can be cataloged, and you just need to find or deduce the list, it's that the rules provide a very general idea of connecting planes and allow for extremely diverse ways of connecting them.

3. The rules aren't built that way

The rules just aren't built ground up to be a perfectly cohesive unified whole that always makes sense. I mean, obviously, right? It's amazing they're as cohesive as they are.

As a Player

You're not going to find the unifying principle that allows you to exploit all of these things, because there isn't one. You can probably find individual cases, like the Portable Hole, that are mildly exploitable.

My best advice is to focus on specific cases that you think you're interested in exploring in your game, do your research, and discuss them with your DM.

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    \$\begingroup\$ +1 I think you hit a key point by calling out that not all portals need to be the same. That also would mean the question about casting through portals is not a duplicate of the one casting into a portable hole: the hole's portal is a specific instance that is not blocking anything, but other portals could work entirely differently. One may not be able to say more about them than you do here, but the answer would be different than that for portable hole. \$\endgroup\$ May 9, 2022 at 10:57
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The rules don't say

Nothing in the rules tells us that the "distance" between points on different planes is even a quantity that makes sense to measure. We shouldn't expect this to be true.

When someone tries to use a spell that crosses a planar boundary, the rules don't tell us clearly whether it works, so the DM makes a ruling. Lots of things in 5e work this way.

You've written:

Do open portals, gates or other interplanar transition points change the distance between planes from infinite (or at least undefined) to 0 at their exact location, and turn distances that can be measured from there into normal, continuous distances?

and, yes, that appears to be a reasonable interpretation, so long as there is a straight line through the portal between the source and destination.

If there's not a straight line through the portal, your DM might rule differently. For example:

  • Suppose I'm at the bottom of a 10ft portable hole, and I want to message to my friend who is 40 feet away horizontally. Is the distance 41.2ft, meaning my message succeeds? Or will my DM rule that the spell fails, because it tries to go in a straight line but there's no portal if you go in a straight line?
  • Suppose I'm standing 100ft away from a gate to the Feywild, and I cast locate object to find my favorite hat. My favorite hat is at the same place I'm standing, but in the Feywild. I could walk that distance using 200ft of movement, but it's up to a DM ruling whether the spell can travel through the gate and do a 180-degree turn to detect my hat.
  • Suppose I try to dimension door 200ft through a gate spell. The dimension door spell doesn't really transport "through" space; can it do planar travel at all? My guess is the DM would rule no.

So maybe we could say:

Magical effects can pass through a gate or other dimensional portal, provided there is a straight line from the source through the portal to the destination. Measure distance in the way that makes sense.

and maybe we would add a special case about "spread" spells such as fireball being able to go around corners.

But please note that this isn't a rule. This is a heuristic which seems to be a good match for the way that we think that a hypothetical DM would rule. That's all you can get from us, because we can't create new rules.

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I question the basis of the question. I don't think there's good support within the rules for the notion that extradimensional spaces are planar in nature; they're just real-space, folded weirdly.

If extradimensional spaces actually existed on other planes of existence and the opening of a bag of holding or portable hole (or rope trick for that matter) was a planar portal or gateway, then I'd expect the interior space to have unusual physical laws, like the Timeless nature of the Astral Plane for example. But they don't. Nothing in the rules says the interior of an extradimensional space is any different from the space outside, it's just bigger on the inside. Extradimensional spaces can open rifts into the Astral because the Astral is what exists outside spacetime and that's where you wind up if you punch a hole through reality.

But can a portal connect two points directly?

All that said, yes, I think an open planar portal or gateway can potentially be described as making two places connect directly, but it does not have to. The DMG describes that some portals exist as a misty passageway from one plane to another (i.e. not zero distance long), and some portals are a location that shifts between planes so there's no measurable distance between the places; you get in the elevator in one plane and get out in another.

Still, some portals can be a simple doorway from here to *there, and the most obvious example of this is a gate spell, which specifically opens a circular hole that "links an unoccupied space you can see within range to a precise location on a different plane of existence". That certainly sounds like you've made a zero-length connection between locations in two planes, but it's really vague enough that the DM may rule that it doesn't work that way.

All it says is "Anything that [moves through the gate] is instantly transported to the other plane, appearing in the unoccupied space nearest to the portal." That does not necessarily mean the portal is a clear and open hole from one place to the other. It doesn't say there is a clear line of sight from one to the other, for example. Your DM might decide that a gate is more like a Stargate, full of a shimmering light, and transit through it is a psychedelic lightshow that lasts a few perceptual seconds while no time passes in the outside world. They might decide that the gate spits you out in a semi-random position relative to the exit gate, regardless of where you were relative to the entry gate, and trying to measure distance from one to the other still results in impossible answers (i.e. all your distance-limited spells and effects fail at the boundary).

Or it may be a clear and open hole that nonetheless does not respect physics, where despite the fact that you can clearly see a target on the far side, your spells don't recognize that there is a path from here to there. Possibly some spells can pass through the portal (firebolt) while others can't (locate object) because some spells throw a quasi-physical object while others specifically only work on the caster's plane of existence.

Ultimately this is a question for your DM. Planar physics are not well defined, and portals even less so, so there isn't a solid rules-as-written answer to how they work.

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  • \$\begingroup\$ Extra-dimensional space; it's kinda in the name, innit? Adding more dimension folding onto a specific part of a plane, not a separate plane itself. I would encourage you to add the first part of this, before the portals, as an answer to [my question[(rpg.stackexchange.com/questions/198195/…). \$\endgroup\$
    – Kirt
    May 9, 2022 at 14:43
  • \$\begingroup\$ @DarthPseudonym, I think you may have a point there too -- both planes and extradimensional spaces are other dimensions, but that does not mean they are the same thing. Planes are typically huge with their own laws of magic, physics, etc, A standard demiplane or portable hole or bag of holding seems to be a plane in the same way a puddle is an ocean. If that is so, however the linked question about if you can cast through portals is not a duplicate of if you can cast into a portable hole (it was closed with the argument they are the same thing). \$\endgroup\$ May 9, 2022 at 16:16
  • \$\begingroup\$ well... I now doubt my answer. The DMG's section on "Demiplanes" (p.68) specifically calls out Mordenkainen's Magnificent Mansion as creating a demiplane, while the spell itself says it's an extradimensional space, which suggests those things are the same and a bag of holding is indeed a very small demiplane. That has the bizarre implication that you could plane shift into a bag of holding if you knew the right frequency, which sounds like the best dumb assassination plot ever. Or the DMG could be wrong and conflating two similar but different concepts. YMMV. \$\endgroup\$ May 9, 2022 at 16:53
  • \$\begingroup\$ The thing about the Mansion is that it is both huge and allows you to create local effects in a way that a bag or rope trick do not. I would say that the Mansion is both an extradimensional space and a demiplane - not because those are the same thing, but because it has aspects of both. Same for a genie warlock's vessel. While a haversack is only an extradimensional space and Ravenloft is only a demiplane as it is 'unmoored' and not an extra folding on another plane. \$\endgroup\$
    – Kirt
    May 9, 2022 at 17:00
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Depends on the Portal

Portals that (meta)physically connect "there" to "here" could reduce the distance between planes to zero. Of course, one-way portals will only do that in one direction. Or they might not, check with your DM.

Portals that allow teleport-like transport between "there" and "here" maintain that distance except for things that can pass through the portal. Some portals might transport creatures (with or hilariously without their equipment), non-living objects, magic or any combination thereof.

I think the problem you have is that you are using the word portal thinking it means something like "Toyota Carolla" whereas it actually means "motor vehicle" or, actually, "means of transportation". Think about the following portals and how different they are:

Some of these bridge the distance, some don't.

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  • \$\begingroup\$ That's a wonderful list, thank you! \$\endgroup\$ May 10, 2022 at 7:21
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I would argue that A is incorrect, as the planes are generally discrete from each other, not continuous.

"No measurement possible" is not the same as "Infinite".

With that, I would argue that an open portal would possibly allow a measurable distance, but that would also depend on what game mechanic needed the measurement, and whether the specific type of portal allowed a line of effect.

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