I originally wrote my ode to the Gygaxian multiverse here:
www.enworld.org
And in it, I made a passing reference to how the true Gygaxian multiverse, with a strong emphasis on the design space of the multiple worlds/realms/planes within the Prime Material Plane, ended, pointing to the time of the Manual of the Planes and Planescape.
Now I'm going to be more explicit as to when this change occurred, and why I think it matters. But first, a detour!
A. Timey wimey vs. Primer.
There is all sorts of time travel in fiction. I would say that, for the most part, there are two extremes of time travel when it comes to the type of fiction that we all know and love. On the one end, we have time travel as we see in Doctor Who. There's a lot of time travel in Doctor Who! And sure, sometimes issues like "paradoxes" and "fixed points in time" can matter a great deal. But .... most of the time, it's really all just a bunch of "timey wimey" stuff that lets the protagonists zip around in time and space and have some fun. The rules are kind of fuzzy and malleable and can change from season to season, episode to episode.
At the other extreme is a movie like Primer, where the mechanics of time travel are integral to the plot. The movie itself is an intricate puzzle ...um.... box.... that requires a strict application of that universe's application of its rules of time travel to function.
The reason I bring this up is not to continue my rant about how "Too much time emphasis on travel in Star Trek ruins the show" (although it does), but instead to say that these different approaches both have something to recommend them, but they result in very different experiences; they both have time travel, but function follows form. The nature of the fiction is very different due to the way that they are using time travel. It's much the same with the diminution of the Gygaxian Prime Material Plane.
B. Grubb's Folly, or how the Manual of the Planes Changed the Overton Window of D&D.
In the prior post, I said that the brilliance of the Gygaxian model was the emphasis it placed on the Prime Material Plane ("PMP"). References to the outer planes abound, from the Paladin in Hell in the PHB to a reference to a trip to the abyss in the DMG; but the thrust of the design space was in these strange alternate realities. In fact, it was assumed that all campaigns would invoke these alternate realities at some point! In the DMG, in a section called The Ongoing Campaign, Gygax specifically states that after a few months or a year, people will grow bored of castles and dungeons and fantasy wilderness, and that's when you should plug in something fun- alternate realities (Alice in Wonderland) or Gamma World, or Boot Hill, or whatever.
This was in accord with the general trend in the published material; it was not uncommon to have characters either "cross genres" (Barrier Peaks) or cross worlds (Amber, Demonweb, Alice, etc.). It was commonly acknowledged that the barriers between realms in the PMP could be, and would be, breached, both in the variously-named Gygaxian worlds (Oerth, Aerth, Earth, Uerth, Yarth, etc.) or when Elminster started pontificating in Dragon magazine.
But this changed with the Manual of the Planes. While the outer planes had been touched on previously in Dragon Magazine (see, for example, Dragon #67, November 1982, which provided for the framework for Astral Plane travel and encounters, or Dragon #73, which was .... well, confusing), the Manual of the Planes was the first core rulebook to really flesh out the planar structure; sure, Deities and Demigods had to address it a little, but this was going to the ur-source book moving forward.
.... and that was the problem. Look, Manual of the Planes had a cool cover. And there were bits and bobs that were pretty neat! But look at the structure- you have 116 pages, and then, finally, you get to page 117, Appendix 1, before it goes into any detail about the Prime Material Plane. And there are three, short pages. And in those three, short pages, the following is done:
1. It changes the established paradigm of what the PMP is. Prior to this book, the PMP was a singular plane encompassing infinite realms (worlds, planes). Now, there were infinite Prime Material Planes, each with their own Ethereal Plane, but the same inner planes and outer planes. This seems like a small distinction (one vessel containing infinite planes v. infinite separate planes) but it led to the next change, which is ....
2. It explicitly made traveling between alternate planes within the PMP more difficult. After all, if they are separate, then it would be more difficult, right! Of course, it stated that they all share the same inner and outer planes, and never fully explained how you'd travel to the inner plane and get back to your own PMP, but that's neither here nor there. And then ....
3. It created a system of arbitrary classifications (physical factor, magical factor, and temporal factor) that, as far I knew, were not used in future publications, to "classify" all alternate PMPs.
The net effect of these changes is what we see today; the loss of the use of the "alternate PMP" as a primary design space, and the codification of the use of the outer planes for "weird" or "kooky" design (memorably in settings such as Planescape). Moreover, it had the effect of the increased "silo-ing" of campaign worlds and the reification of a more static, less weird multiverse.
C. What does it mean, though, really? More fundamentally, who cares? And what was up with that timey wimey stuff?
Let's use an easy example. In the Gygaxian multiverse, Ravenloft is released as a module in 1983. Fog rolls in and traps the adventurers in Barovia? Sure, it can just be an alternate place. Probably seen it before. After all, by that time, who wasn't familiar with portals to Averoigne, or portals to Dungeonland, or the many portals that Lolth was using to invade other Prime Material Planes.
Post-Gygax, of course, there has to be an outer plane, a Dread plane, in order to have that type of adventure of campaign. On the one hand, you could say that this doesn't change anything; what is the difference between an alternate prime material plane and a demiplane? But on the other hand, it is different. Because function follows form; the easy standard that we once had (as seen in the other article, with the linked-to example from Jim Ward in 1974, having three doors!) for quickly and easily changing genres, rules, and tone are now calcified and hardened into stronger rules.
Which can always be changed in the home campaign, but the focus leads to a dearth of excellent published material because, in my opinion, they no longer use the design space that they have with the PMP. To go back- I love Primer. Primer is wonderful. But there are a lot of times when I'd rather just sit back and have a good time, and go between horror, and comedy, and the past, and the future, and have a good time without worrying too much about the underlying mechanics. Re-opening the full design space a la the Gygaxian multiverse for the Prime Material Plane would still preserve everything people love about the outer planes, and still allow people to run gritty fantasy, while re-opening the game to a degree of weirdness.

D&D General - The Brilliance of the Original Gygaxian Multiverse
Good artists borrow, great artists steal. I was looking at the following conversations in these two threads about the multiverse in D&D, and how that certain Card Game's setting can be incorporated into the D&D multiverse, and, for that matter, how the multiple D&D settings that we are seeing...

And in it, I made a passing reference to how the true Gygaxian multiverse, with a strong emphasis on the design space of the multiple worlds/realms/planes within the Prime Material Plane, ended, pointing to the time of the Manual of the Planes and Planescape.
Now I'm going to be more explicit as to when this change occurred, and why I think it matters. But first, a detour!
A. Timey wimey vs. Primer.
There is all sorts of time travel in fiction. I would say that, for the most part, there are two extremes of time travel when it comes to the type of fiction that we all know and love. On the one end, we have time travel as we see in Doctor Who. There's a lot of time travel in Doctor Who! And sure, sometimes issues like "paradoxes" and "fixed points in time" can matter a great deal. But .... most of the time, it's really all just a bunch of "timey wimey" stuff that lets the protagonists zip around in time and space and have some fun. The rules are kind of fuzzy and malleable and can change from season to season, episode to episode.
At the other extreme is a movie like Primer, where the mechanics of time travel are integral to the plot. The movie itself is an intricate puzzle ...um.... box.... that requires a strict application of that universe's application of its rules of time travel to function.
The reason I bring this up is not to continue my rant about how "Too much time emphasis on travel in Star Trek ruins the show" (although it does), but instead to say that these different approaches both have something to recommend them, but they result in very different experiences; they both have time travel, but function follows form. The nature of the fiction is very different due to the way that they are using time travel. It's much the same with the diminution of the Gygaxian Prime Material Plane.
B. Grubb's Folly, or how the Manual of the Planes Changed the Overton Window of D&D.
In the prior post, I said that the brilliance of the Gygaxian model was the emphasis it placed on the Prime Material Plane ("PMP"). References to the outer planes abound, from the Paladin in Hell in the PHB to a reference to a trip to the abyss in the DMG; but the thrust of the design space was in these strange alternate realities. In fact, it was assumed that all campaigns would invoke these alternate realities at some point! In the DMG, in a section called The Ongoing Campaign, Gygax specifically states that after a few months or a year, people will grow bored of castles and dungeons and fantasy wilderness, and that's when you should plug in something fun- alternate realities (Alice in Wonderland) or Gamma World, or Boot Hill, or whatever.
This was in accord with the general trend in the published material; it was not uncommon to have characters either "cross genres" (Barrier Peaks) or cross worlds (Amber, Demonweb, Alice, etc.). It was commonly acknowledged that the barriers between realms in the PMP could be, and would be, breached, both in the variously-named Gygaxian worlds (Oerth, Aerth, Earth, Uerth, Yarth, etc.) or when Elminster started pontificating in Dragon magazine.
But this changed with the Manual of the Planes. While the outer planes had been touched on previously in Dragon Magazine (see, for example, Dragon #67, November 1982, which provided for the framework for Astral Plane travel and encounters, or Dragon #73, which was .... well, confusing), the Manual of the Planes was the first core rulebook to really flesh out the planar structure; sure, Deities and Demigods had to address it a little, but this was going to the ur-source book moving forward.
.... and that was the problem. Look, Manual of the Planes had a cool cover. And there were bits and bobs that were pretty neat! But look at the structure- you have 116 pages, and then, finally, you get to page 117, Appendix 1, before it goes into any detail about the Prime Material Plane. And there are three, short pages. And in those three, short pages, the following is done:
1. It changes the established paradigm of what the PMP is. Prior to this book, the PMP was a singular plane encompassing infinite realms (worlds, planes). Now, there were infinite Prime Material Planes, each with their own Ethereal Plane, but the same inner planes and outer planes. This seems like a small distinction (one vessel containing infinite planes v. infinite separate planes) but it led to the next change, which is ....
2. It explicitly made traveling between alternate planes within the PMP more difficult. After all, if they are separate, then it would be more difficult, right! Of course, it stated that they all share the same inner and outer planes, and never fully explained how you'd travel to the inner plane and get back to your own PMP, but that's neither here nor there. And then ....
3. It created a system of arbitrary classifications (physical factor, magical factor, and temporal factor) that, as far I knew, were not used in future publications, to "classify" all alternate PMPs.
The net effect of these changes is what we see today; the loss of the use of the "alternate PMP" as a primary design space, and the codification of the use of the outer planes for "weird" or "kooky" design (memorably in settings such as Planescape). Moreover, it had the effect of the increased "silo-ing" of campaign worlds and the reification of a more static, less weird multiverse.
C. What does it mean, though, really? More fundamentally, who cares? And what was up with that timey wimey stuff?
Let's use an easy example. In the Gygaxian multiverse, Ravenloft is released as a module in 1983. Fog rolls in and traps the adventurers in Barovia? Sure, it can just be an alternate place. Probably seen it before. After all, by that time, who wasn't familiar with portals to Averoigne, or portals to Dungeonland, or the many portals that Lolth was using to invade other Prime Material Planes.
Post-Gygax, of course, there has to be an outer plane, a Dread plane, in order to have that type of adventure of campaign. On the one hand, you could say that this doesn't change anything; what is the difference between an alternate prime material plane and a demiplane? But on the other hand, it is different. Because function follows form; the easy standard that we once had (as seen in the other article, with the linked-to example from Jim Ward in 1974, having three doors!) for quickly and easily changing genres, rules, and tone are now calcified and hardened into stronger rules.
Which can always be changed in the home campaign, but the focus leads to a dearth of excellent published material because, in my opinion, they no longer use the design space that they have with the PMP. To go back- I love Primer. Primer is wonderful. But there are a lot of times when I'd rather just sit back and have a good time, and go between horror, and comedy, and the past, and the future, and have a good time without worrying too much about the underlying mechanics. Re-opening the full design space a la the Gygaxian multiverse for the Prime Material Plane would still preserve everything people love about the outer planes, and still allow people to run gritty fantasy, while re-opening the game to a degree of weirdness.