Namespaces add a considerable amount of complexity to processing and pose two large challenges to schema languages. The first challenge to address is to associate namespace URIs with patterns that describe elements and attributes. The solution to this challenge will be described in the next section. The second issue is to provide mechanisms for creating extensible schemas for documents that may contain content in multiple namespaces. Of course, writing extensible schemas is an issue that goes beyond multinamespace documents; you will see more use of that in the next chapter. In this chapter, I will introduce name classes, the key to namespace-based extensibility with RELAX NG.