A Document Introduction When namespaces are introduced into a document, you may not always get what you think. You need to be able to figure out what namespaces are from where. While there are tools, they may not always be correct. You should be able to figure out what is going on by hand. That is you should be able to do: Understand the scope of a prefix. Know whether namespace names are equivalent. Be able to list the in-scope namespaces for any element. Conclusion Even so, you namespaces are tricky. You may spend way too much time tracking down a problem that ends up being either a bad namespace declaration or an incorrect assumption about the namespace. After many years, I still run into this problem!