There are nine realms, sometimes called nine homeworlds, but there isn't a single surviving list of what they are. Instead it's left up to us to sort them out.
There is a list of six groups each of which have a homeworld (Humans, Aesir, Vanir, Jotuns, Elves, Corpses) to this we can add the Black Elves, also known as Dwarves, and get seven. Add in the two primordial realms and we get nine.
And that is the traditional listing, using the order from above we get
- Midgard - Home to humans
- Asgard - Home to the Aesir gods
- Vanaheim - Home to the Vanir gods
- Jotunheim - Home to (many of) the Jotuns
- Álfheim - Home to the (Light) Elves
- Helheim - Home to dead people, the ones not in Valhalla.
- Svartálfaheim - Home to the Black Elves also known as Dwarves.
- Niflheim - Primordial realm of ice and snow and mist and stuff
- Muspellsheim - Primordial realm of fire and lava and such.
The major problem here is that Hel, who created Helheim, wasn't sent into the mists of "never been a realm here" when Odin
She was sent somewhere that already existed and set up shop there. Now it's possible that she created a world that broke off from the previous one and became a realm unto itself (where do you think most of the nine came from?) but it doesn't just say she was cast into an existing realm, Niflheim, it says that here kingdom is there. Implying that Helheim is not a ream unto itself but just a location within greater Niflheim.
So there's one problem, our list of nine might actually only be a list of eight. Another problem is the question of how they relate to each other.
* * *
There's not a lot of information to sort them out.
Niflheim and Múspellsheim are opposite each other. That helps. All we need to do is locate one of them and we automatically know where the other one is.
Midgard is in the middle. That's useful.
The home of the Dwarves/Black Elves is below Midgard.
The (Light) Elves are up. They're also south of Asgard but that doesn't really help because I don't think we have north/south east/west directions on Asgard itself.
The world tree has three roots and ... oh my gods do they not make sense.
One goes to a well in Jotunheim. At least that much is clear. Before it touches down and meets the well, it goes over Jotunheim. Makes sense.
Helheim is under another of the roots. Midgard under the last. Would be nice if the roots had names.
We also know that the roots each go to a well. Already covered the well in Jotunheim. The other two are in Niflheim and Asgard.
Given the placement of Helheim inside of Niflheim we can probably assume that one over Helheim goes to the well in Niflheim.
So, then, the one that goes over Midgard would be, process of elimination, the one that goes to the well in Asgard which we are to interpret . . . how?
Roots often go down, so we could interpret the fact that Asgard is at the end as meaning that it is below Midgard. On the other hand, if it is a more horizontal root then it being ground level in Asgard but above Midgard, then Asgard would be higher.
And then there's this. While Gylfaginning clearly places the well the root taps as residing among the gods, they have to cross the Bifrost (rainbow bridge) daily to reach it. The Bifrost is the bridge connecting Midgard and Asgard. So, if the gods live in Asgard, and the well is with them, why do they have to cross the bridge?
Maybe the bridge has multiple stops?
And then there's a question of what it means to go "up" a rainbow. Rainbows go in circles. Go up for long enough and you'll be going level, longer still and you'll be going down. The fact that Asgard is up the rainbow bridge from Midgard doesn't mean it's above Midgard. In fact it sort of makes more sense for it to be below.
You can, after all, see the full arc of a rainbow under the right conditions, but earthbound humans can never see the full circle. If going up one side is how you get to asgard, and you can't see asgard from here, then you must have to go over the arch or the rainbow, down the other side, and below the horizon.
The whole thing is a mess to interpret.
Let this be a call to action. What do you believe? Whatever it is, write it down in great detail, make many copies, seal most of them against the elements, and bury the sealed ones in deserts and/or peat bogs, and try to circulate the rest widely. Future historians and mythographers will be grateful.
* * *
Recap of what we know: Midgard is in the middle. Helheim is in Niflheim.
At the roots are Asgard, Jotunheim, and Helheim/Niflheim
Svartálfaheim is down, Álfheim is up.
Muspellsheim is opposite Niflheim.
* * *
Trying to synthesis and also making shit up:
The simple directions of the two Elf homworlds suggests to me that Svartálfaheim--Midgard--Álfheim represents an axis along the trunk.
Svartálfaheim is underground, it can't be too far underground because tree roots tend to stay close to the surface, additionally Yggdrasil's main roots are not some extension of a young taproot that was somehow encouraged to continue as a straight down trunk extension. Still, we're talking Yggdrasil here; not too far underground for Yggdrasil is probably enough space to fit a really nice realm or two.
So I'm saying that Svartálfaheim is under Yggdrasil's trunk,
Midgard is at "ground" level of the trunk. That's pretty much accepted. Mind you what "Ground means for a tree connecting worlds is ... unclear at best.
Álfheim is straight up the trunk.
Three along the trunk and three in the roots leaves us with three left to determine.
Muspellsheim is easy. It's opposite Midgard from Niflheim, which is in the roots, so it would be in the branches on the side non-Niflheim side of the tree. (How can the realm of lava be elevated? I don't think it's a problem for Yggdrasil. Yggdrasil can probably have a realm of endless empty space at the bottom most portion of its deepest root.)
We're left to determine the location of Vanaheim and ????. Putting together Helheim and Niflheim has left us a realm short. At this point I abandon all pretense of logical reasoning (and openly so, unlike before when I didn't point it out) and draw on aesthetics and analogy.
The fire giants live somewhere. But Muspelleheim doesn't seem like a place to live. The frost giants don't live in Niflheim. Primordial realms are not good places to live. Note that the inhabitants of Helheim, located within Niflheim, are all dead.
If the frost giants don't dwell in the realm of primordial realm of frost but instead somewhere off a different root over to the side somewhere, then by analogy the fire giants shouldn't dwell in the primordial realm of fire but instead in a different realm off a different branch off to the side somewhere.
So I posit their world as the ninth realm they are the Eldjötnar, so their world would be Eldjotenheim. I place it opposite Jotunheim. (Note that there is president for [something]heim and prefix[something]heim both existing, see Álfheim and Svartálfaheim.)
The only realm not yet placed is Vanaheim, and the only asthetically pleasing place to put it is in the branches opposite Asgard.
This leaves us with three realms in the branches (Muspelleheim, Eldjotenheim, and Vanaheim), three along the trunk (Svartálfaheim, Midgard, and Álfheim) and three at the roots (Asgard, Jotunheim, and Niflheim which contains Helheim.)
It also leaves us with the following oppositions:
Álfheim is opposite Svartálfaheim
Jotunheim is opposite Eldjotunheim
Asgard is opposite Vanaheim
Niflheim is opposite Muspelleheim
Midgard is in the middle.
And that's the model of the Nine Realms I come up with. Given that I had to make up a name "Eldjotunheim" to get there, I'm, pretty sure no one actually follows or believes in this model. But it makes sense to me.