x-default
TL;DR
x-default is a special hreflang value indicating which page Google should show when no other language variant fits the searcher.
x-default is the 'fallback' signal in your hreflang setup. For a Dutch site with NL, EN and DE variants you specify via x-default which version a searcher from Spain, Brazil or Japan sees (usually English). Without x-default Google guesses.
Example
<link rel="alternate" hreflang="x-default" href="https://webrockmedia.nl/"> tells Google: 'show the Dutch version to unknown languages/regions'.
Frequently asked questions
Is x-default mandatory?
No, but strongly recommended. For most international SEO setups it prevents ambiguity.
Related terms
Further reading
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