What is a canonical URL?
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Canonical URL: The preferred address for a page when the same or very similar content is reachable at more than one URL, so search engines consolidate signals there.
You set it with a rel="canonical" link in the page head or an HTTP header, and a redirect says the same thing more strongly. Google treats these as signals, not commands, and picks a canonical itself when none is given. The link relation was written up as RFC 6596 in 2012.
AI search tools that lean on a search index inherit its choice, so the version of a page that answers in ChatGPT or Claude is usually the canonical one. An XML sitemap should list canonical URLs only.
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