Namespaces can be both simple and complicated. The very first example schema in this book included an attribute from the xml:lang namespace and it didn't seem like a big deal. However, if you think about it more carefully, you'll see that namespaces present two different challenges to schema languages. The first is that schema languages need to provide a way to define which namespaces belong to the elements and attributes which are described; the second is how schema languages can cope with extensibility, one of the objectives of XML namespaces.

In this chapter, we'll take a closer look at these two challenges before seeing how RELAX NG addresses them.