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Acheron is between pure law and lawful evil; Gehenna is between pure evil and lawful evil; and Carceri is between pure evil and chaotic evil.

But this seems wrong to me. It seems to me that Acheron should be where Gehenna is, Gehenna should be where Carceri is, and Carceri should be where Acheron is.

After all:

• Acheron is an endless battlefield with constant bloodshed: to me, that rings of pure evil with only a bit of law thrown in.




• Gehenna is a series of gigantic volcanoes constantly blowing up and shooting magma everywhere. The entire plane has 45-degree slopes everywhere. Seems to lean towards chaos, seeing as how fire is falling from the sky, rocks are sliding down at you, and you're always in danger of slipping down the mountainside.




• Finally, Carceri is the one that seems the most out of place. It's a gigantic prison. Prisons have rules and a hierarchy of guards, jailors, and wardens. I would think a prison would be extremely lawful.

Does the lore provide an explanation as to why this is the way it is? The rest of the Great Wheel seems to work fine; it's just these three evil planes which seem off. I'm mostly interested in the Great Wheel specifically, but if there are other D&D cosmologies that have those planes in a similar setup with respect to alignment, and those have some sort of explanation, that would also be useful.

I've made a post on the GitP forums for further speculation about this topic.

Noted in your forum thread, but the best descriptions of Acheron and Carceri that I have seen—based in, but not directly quoted from, actual text—are those by Jade Ripley (who goes by Lord_Gareth here and numerous other places, including GiantITP.com’s forums):

• 
  

  Acheron

  
  
  

  
  

  Acheron has no architects. None of the Outer Planes do. [...]

  
  
  

  Yes, Acheron looks a little Chaotic on the outside. You know what, from the outside Carceri looks a little lawful. But Acheron isn't chaotic. Acheron is battle without resolution, law without harmony, order without structure, misery without hope, death without glory, unity without individuality. Acheron is not a plane that hates you; indeed, it is Acheron's utter indifference to you that eventually kills you. Acheron is the grinding monotony of hopelessness, and it is the weary horror of cynicism so great that it consumes morality. The sergeant who grows weary of fighting corruption and embraces bribery goes to Acheron; the office drone who takes out his misery on others by providing them barriers to actual help goes to Acheron. It is the punishment for which there was no crime, the penalty without a violation, the monolithic crushing indifference of Law with no moral compass, of conflict without belief, of tyranny without vanity.

  
  
  

  Acheron doesn't hate you.

  
  
  

  It wants you to die anyway.

  
  

  
  


• 
  

  Carceri

  
  
  

  
  

  Carceri, the Red Prison, is a plane of injustice. Carceri is a mockery of law and lawful thinking, seen through a lens of powerful and self-defeating malevolence. Carceri is full of vain struggle, wretched anguish, trapped rage, and a heady mix of hope and despair that drives its inhabitants mad. From the outside, Carceri looks lawful - it is a prison, after all - but the thing that separates Carceri from ordinary prisons is that no one is in charge, and despite delusions to the contrary no one can ever be in charge. Though the denizens of Carceri make some effort to separate 'prisoners' from people who 'just live there' (that is, who are not locked up in a specific prison complex, prison structure, or torment) the truth is that all being in Carceri are prisoners, trapped there by their own fear, hate, mistrust, paranoia, vices, greed, desires, and malice.

  
  
  

  And, really, that's the thrust of Carceri. The Red Prison mocks you with hope, offering escape and giving only frustration and despair. Even if you leave, it drags you right back, proving that any escape is merely temporary. Its "wardens" cannot leave, and they cannot impose order on their prisoners any further than the reach of their weapons. Riots and murder abound, a seething mass of hate and frustration that shakes the bars and rattles the cages, echoing through the plane and mixing with the shrieks of pain and pleasure. Really, it's not the plane keeping you there, it's you.

  
  
  

  It was always you. It's Carceri's greatest and cruelest irony. In the end, the reason you will never leave is that even if you were fit for society once, you never will be again. You can leave Carceri, but Carceri, it never leaves you.

  
  

  
  

As for Gehenna, the landscape is rather uniform: a constant slope, a constant rain of fire and rock, and a constant absence of much of anything else. It is a barren landscape, and that uniformity is fairly lawful.

But always remember that no one knows what Law and Chaos really mean. They’re nebulously defined, and their relationship to landscape in particular is not often discussed in the books—Acheron, Carceri, and Gehenna are those alignments because those are the alignments the authors gave it, and if those authors have ever explained their choices, I’m not aware of it. Even if they have, their considerations are going to be highly personal and idiosyncratic—because everyone’s definitions of Chaos and Law are. There is a reason no one ever published a Book of Unfettered Discord or Book of Perfect Dogma—no one can pin down Chaos and Law well enough to write it. It’s all “I know it when I see it,” except everyone keeps disagreeing on when they’ve seen it. So ultimately, it’s unsurprising that these realms don’t seem to match their alignments to you, because your conception of those alignments is yours and more-or-less yours alone. D&D has a conceit wherein these alignments are objective, universal truths—but the real world doesn’t.

Primarily, this is based on precedent

Each of these planes was introduced in the article "Planes: The Concepts of Spatial, Temporal and Physical Relationships in D&D", in The Dragon #8, released July 1977. The alignments of these planes seem to have been set way back then and have been generally maintained with some minor alterations. There are good reasons for each classification, expounded upon below. I have gotten my primary information from Wikipedia articles with sources of their own and have linked to each in the subtitles below.

Acheron

This one focuses mostly on hierarchical structure in regards to the endless war.

It is a plane of law where conformity takes precedence over any thoughts of good.

The floating cube structures throughout the plane seem to emphasize its rigidity and "lawfulness".

Gehenna

It seems this is mostly due to the relationship with yugoloths, the primary inhabitants. In the appendix of AD&D 1e, it is (ambiguously) called "The furnaces of Gehenna of lawful evil neutrals".

Carceri

The key to understanding the classification of Carceri is the nature of its layers. This is not a prison in the traditional sense; rather, it's a series of six unique wastelands. There are bastions of organization, but it is mostly bleak, elemental chaos.