You can break a typical document into three layers: structure, content, and style. SGML works by separating these three aspects and deals mainly with the relationship between structure and content. At the heart of an SGML application is a file called the DTD, or Document Type Definition. The DTD sets up the structure of a document, much like a database schema describes the types of information it handles. A database schema also defines the relationships between the various types of data. Similarly, a DTD specifies rules to help ensure documents have a consistent, logical structure. Content is the information itself. The method for identifying the information and its meaning within this framework is called tagging. Tagging must conform to the rules established in the DTD (see ). SGML does not standardize style or other processing methods for information stored in SGML.