XML Namespaces

Namespaces stop name clashes when one XML document mixes elements from different sources.

The Problem

Because you choose your own tag names, two people can use the same name for different things. A furniture shop's <table> is not the same as an HTML <table>:

<table>
  <tr><td>Chair</td><td>Rs 3500</td></tr>
</table>

<table>
  <material>Sheesham wood</material>
  <seats>6</seats>
</table>

If both appear in one document, a program cannot tell them apart.

Prefixes

A prefix in front of the name solves it:

<h:table>
  <h:tr><h:td>Chair</h:td></h:tr>
</h:table>

<f:table>
  <f:material>Sheesham wood</f:material>
</f:table>

But prefixes alone are not enough: someone else might also choose h. So each prefix is linked to a unique name.

The xmlns Attribute

You declare a namespace with xmlns:prefix="URI":

<?xml version="1.0" encoding="UTF-8"?>
<catalog
  xmlns:h="http://www.w3.org/1999/xhtml"
  xmlns:f="https://example.com/furniture">

  <h:table>
    <h:tr><h:td>Price list</h:td></h:tr>
  </h:table>

  <f:table>
    <f:material>Sheesham wood</f:material>
    <f:seats>6</f:seats>
  </f:table>
</catalog>

The URI (like https://example.com/furniture) is just a unique name. Programs do not open it, and it does not need to be a real web page. Companies use their own domain so that nobody else uses the same URI.

Default Namespace

If you write xmlns="..." with no prefix, every element without a prefix belongs to that namespace. This keeps files clean:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>https://example.com/</loc>
    <lastmod>2026-10-15</lastmod>
  </url>
</urlset>

This is a real website sitemap. urlset, url, loc and lastmod all belong to the sitemap namespace.

Namespaces in Files You Know

For example, an SVG file starts like this:

<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
  <circle cx="50" cy="50" r="40" fill="orange"/>
</svg>

An orange circle, 80 pixels wide, in the middle of a 100 by 100 image.

Without the xmlns, a browser opening the .svg file would not know these are SVG drawing elements.

Scope

A namespace declaration applies to the element where it is written and to everything inside it. You can declare namespaces on the root (most common) or on any inner element.

Attributes and Namespaces

Attributes without a prefix do not belong to the default namespace; they simply belong to their element. Attributes with a prefix, like android:text="Hello", belong to that prefix's namespace.

Exercise