If you are new to some of these concepts, this discussion might have seemed a bit confusing, so here's the bottom line. The tags in the SGML version of our memo were created specifically for a memo document. That is possible because SGML is extensible, which means that it allows an author to define a particular structure (such as a memo document structure) by defining the parts (tags) that fit that structure. You know that HTML is an application of SGML. That means that a group of people sat down and, using SGML, came up with a whole new language to be used specifically for formatting documents on the World Wide Web. The HTML language, however, is not extensible, which means that HTML cannot be used as a basis to create another markup language with its own rules and purposes. Because SGML allows for the creation of other languages, calling SGML a markup language is a bit misleading. Instead, it's actually a metalanguage that allows for the creation of other languages.
At this point, you might be wondering what SGML has to do with the purpose of this book, which is to teach XML. Well, XML is a direct descendant of SGML. But unlike HTML, XML is not really a new language. It is actually a subset of SGML and also qualifies as a metalanguage. In fact, SGML can do everything that XML can do, and more. But if this is true, why do we need XML? The next chapter answers that question.