When making environment textures you are often limited in your texture size. This means you have two choices, everything can have its own texture space and as a result will be blurry, but you can put unique details everywhere. The other option is to use overlapping mirroring, which will reuse the same portion of the texture on multiple places.
This will increase the total number of pixels on the object so everything will look sharper. Because it is being reused, you have the potential for noticeable repetition which looks bad. One way to blend the two together is a process called "thirding", although you can use it more than 3 times.
You essentially make a single texture that would work on a face, but keep the details that explain the edges confined to the very extreme borders. This way you can map it to multiple faces but keep the "inner" faces to the inside portion, stopping just short of those outside edge borders.
The outer faces need to stop just short of the detail from the opposite end. This will make it seem as if the texture is one long continuous texture.
The only thing you have to worry about is the area shown by the green brackets, is identical, so that it will tile well. The red area needs to be fairly generic so that it will go on 3 times without showing the repetition.
