The reason for introducing tagging into a document is to label and organize it for machine processing. If the paragraphs are clearly marked, then a formatter can lay them out properly. If the place-names are clearly marked, a program can automatically pick them out to make a geographical index. But this can really only be done reliably if we have some control over how tags are introduced into the document and where they appear. XML technology provides this additional level of control by means of what is called a schema, a kind of combined lexicon and grammar for valid XML documents. We noted above that an XML document is said to be well-formed if it respects the syntactic rules of the XML standard. It may, optionally, also be said to be valid if the tags it contains conform to a schema.