If you opened a sitemap file and saw <sitemapindex> instead of the <urlset> you expected, this is why — and what you can and can't do with it.
Three Names, One Concept — Mostly
"Sitemap index file," "sitemap index," and "sitemapindex" get used interchangeably, and almost always refer to the same idea: a file that lists other sitemap files instead of listing pages directly. The one place the wording actually matters is the XML itself — sitemapindex (no space) is the literal root tag name, and it's case- and spelling-sensitive. Write it as <sitemap index> or <SitemapIndex> and a parser will reject it.
The Tag Swap That Trips People Up
A normal sitemap and a sitemap index use two completely different, non-interchangeable tag pairs:
| A sitemap (lists pages) | A sitemap index (lists sitemaps) | |
|---|---|---|
| Root tag | <urlset> | <sitemapindex> |
| Entry tag | <url> | <sitemap> |
| <loc> points to | A page URL | Another sitemap file |
The mistake that actually breaks things: nesting a <url> block inside a <sitemapindex> (or vice versa). A parser expecting one entry type and encountering the other will throw a validation error — the file needs to be entirely one format or the other, never both.
A Minimal <sitemapindex> File
<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<sitemap>
<loc>https://example.com/sitemap-pages.xml</loc>
</sitemap>
</sitemapindex>Note it uses the same sitemap namespace as a regular sitemap — the namespace doesn't change between the two formats, only the tag names inside it do.
Do You Actually Need One?
Only if you have more than 50,000 URLs, or you want to split your sitemap by content type for easier auditing. For the full decision — including which platforms generate one automatically — see what a sitemap index file is and when you need one. For the complete tag syntax of a regular sitemap, see the XML sitemap format reference.