Internal DTD vs External DTD

Estimated study time: 12 minutes.

A DTD can live in one of two places: directly inside the XML document (internal), or in a separate file referenced by the document (external).

Internal DTD

Declared inside the document's own <!DOCTYPE> block, using square brackets.

<!DOCTYPE course [
  <!ELEMENT course (title, duration)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT duration (#PCDATA)>
]>
<course>
  <title>Data Analytics</title>
  <duration>4 Months</duration>
</course>

Good for small, self-contained documents where the rules only apply to that one file.

External DTD

Stored in a separate .dtd file and referenced by URL or path.

<!DOCTYPE course SYSTEM "course.dtd">
<course>
  <title>Data Analytics</title>
  <duration>4 Months</duration>
</course>

<!-- course.dtd -->
<!ELEMENT course (title, duration)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT duration (#PCDATA)>

Choosing Between Them

• Internal — quick, self-contained, good for one-off or small documents.
• External — reusable across many documents, easier to maintain a single source of truth for shared structure rules.