A DTD Example

<?xml version="1.0"?>
<!DOCTYPE BOOK [
    <!ELEMENT p (#PCDATA)>
    <!ELEMENT BOOK         (OPENER,SUBTITLE?,INTRODUCTION?,(SECTION | PART)+)>
    <!ELEMENT OPENER       (TITLE_TEXT)*>
    <!ELEMENT TITLE_TEXT   (#PCDATA)>
    <!ELEMENT SUBTITLE     (#PCDATA)>
    <!ELEMENT INTRODUCTION (HEADER, p+)+>
    <!ELEMENT PART         (HEADER, CHAPTER+)>
    <!ELEMENT SECTION      (HEADER, p+)>
    <!ELEMENT HEADER       (#PCDATA)>
    <!ELEMENT CHAPTER      (CHAPTER_NUMBER, CHAPTER_TEXT)>
    <!ELEMENT CHAPTER_NUMBER (#PCDATA)>
    <!ELEMENT CHAPTER_TEXT (p)+>
]>
<BOOK>
    <OPENER>
        <TITLE_TEXT>All About Me</TITLE_TEXT>
    </OPENER>
    <PART>
        <HEADER>Welcome To My Book</HEADER>
        <CHAPTER>
            <CHAPTER_NUMBER>CHAPTER 1</CHAPTER_NUMBER>
            <CHAPTER_TEXT>
                <p>Glad you want to hear about me.</p>
                <p>There's so much to say!</p>
                <p>Where should we start?</p>
                <p>How about more about me?</p>
            </CHAPTER_TEXT>
        </CHAPTER>
    </PART>
</BOOK>