Document Structure

Every XML document follows the same basic skeleton: an optional declaration line at the very top, followed by exactly one element that wraps everything else in the document.

The XML declaration

Most XML documents open with a declaration stating the XML version and character encoding in use:

<?xml version="1.0" encoding="UTF-8"?>

This line is technically optional — a parser will assume XML 1.0 and UTF-8 if it's missing — but it's considered good practice to include it explicitly. If you do include it, it has to be the very first thing in the file, with nothing (not even a blank line) before it.

Exactly one root element

Everything else in the document has to sit inside a single top-level element, usually called the root element. A document with two elements sitting side by side at the top level is not valid XML, even though each one individually looks fine:

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book>Dune</book>
  <book>Foundation</book>
</library>

<?xml version="1.0" encoding="UTF-8"?>
<book>Dune</book>
<book>Foundation</book>

error: XML document structures must start and end within the same entity. (extra content at the end of the document)

The second example has two <book> elements sitting at the top level with nothing wrapping them — a parser reads the first <book>...</book> as the complete document and then finds unexpected content immediately after it, which it treats as an error rather than a second top-level element.

Nesting inside the root

Once you have a root element, everything else nests inside it to whatever depth the data actually needs — a root can contain children, and those children can contain their own children:

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book>
    <title>Dune</title>
    <author>
      <name>Frank Herbert</name>
      <nationality>American</nationality>
    </author>
  </book>
</library>