x-default is the hreflang fallback signal. When a search engine returns results for a user whose preferences don't match any of the page's other hreflang= entries, it serves the URL marked x-default instead.
Common use cases for the x-default URL:
- A language-selection landing page that asks the user to pick.
- The site's "primary" or "international" version (often the English / unlocalized variant).
- A regional default for users whose country isn't separately localized.
Without x-default, search engines fall back to a heuristic -- usually returning whichever variant they think is closest based on rough geo-IP and Accept-Language matching. The heuristic is unpredictable and operators rarely like the result.
Google specifically recommends including x-default for any international site -- it's "free" SEO hygiene with a clear behavioral payoff. The BeaverCheck hreflang analyzer flags its absence as Info-severity for any page with hreflang tags but no x-default entry.
Note: x-default doesn't follow BCP 47 -- it's an hreflang-specific keyword. The BCP 47 validator is correctly relaxed to accept it.