<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>