There are many reasons why a player would go to the Nether. They can go there for gathering a big amount of Lava, mining Netherite, collecting Soul Sand for a Soul Sand elevator.
Or they can go to build a Wither Skeleton farm, which is a necessary step to take before fighting the Wither, one of the Minecraft bosses, and then being able to build a Beacon (related topic: Potion of Decay).
And if you want to do so, you first need to understand what the Nether Fortress bounding box is, otherwise you won’t even be able to start. In this article we are going to see how mobs spawn in Nether Fortresses, what a bounding box is and how to see bounding boxes in-game.
Note that the concept of bounding box matters exclusively in the Java version of Minecraft, other rules are in place on other platforms.
What is a bounding box?
A bounding box is an invisible parallelepiped which can contain different Minecraft automatically generated structures. Each structure is composed of pieces (like a corridor for Nether Fortresses), which also have their own bounding box and can spawn multiple other pieces attached to them (this is not true for every structure type. It is true for Nether Fortresses though).
When new chunks of the map are generated, structures generate inside bounding boxes and later on specific mobs will spawn inside them.
How mobs spawn in Nether Fortresses
Wither Skeletons are one of those Minecraft mobs that are bounded to a specific structure, like the Iron Golems who can only spawn in Villages. Wither Skeletons, like Blazes, Skeletons, Zombified Piglins and Magma Cubes (cubes, not Magma blocks), fall in the cathegory of Nether Fortress mobs, and in order to spawn they need to fulfill at least one of two requirements:
- the spawn coordinates need to be in a Nether Fortress bounding box and the ground needs to consist of Nether bricks
- the spawn coordinates need to be in the bounding box of a piece of the Fortress (like a corridor, for example)
This is everything that Nether Fortress mobs need to spawn, since light level is not important in the Nether (contrary to the Overworld). The only exception are Blazes, that will also need a Spawner (related: How to find ALL Mob Spawners in Minecraft).
How to see a bounding box
If you want to see bounding boxes (which can be very useful if you want to build a Wither Skeletons farm) you can’t do it without mods. There are different mods that allow you to do so, one of the best choices is probably Bounding Box Outline Reloaded, which you can download here.
Bounding box video
In this video you can see how a bounding box looks like and the logics behind Nether Fortress mobs spwning.