<result><topic topicid="top4">
 <title>Structure</title>
 <para>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.</para>
 <para>A database schema also defines the relationships between the
 various types of data. Similarly, a DTD specifies <emph>rules</emph>
 to help ensure documents have a consistent, logical structure.</para></topic></result>