1. **Internal DTD Example:**
```xml
<!DOCTYPE note [
<!ELEMENT note (to, from, heading, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
```
2. **External DTD Example:**
```xml
<!DOCTYPE book SYSTEM "book.dtd">
<book>
<title>XML Developer's Guide</title>
<author>John Doe</author>
<year>2023</year>
<price>44.95</price>
</book>
```
3. **HTML DTD Example:**
```xml