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Canonicals & duplicate content

Cross-domain canonical

A cross-domain canonical is a rel="canonical" link that points to a URL on a different domain. It tells Google that the preferred version of the content lives there. Like any rel="canonical" annotation, it is a strong signal, and Google may still choose another URL.

When is it used?

The typical case is content that appears on two domains, such as an article a partner republishes. Google’s documentation says the canonical element is not recommended for syndication, because the syndicated pages are often very different. Instead, Google says syndication partners should block indexing of the syndicated content.

Cross-domain canonicals are also a known sign of hacked sites. Google describes attacks that insert a cross-domain canonical pointing to spammy content into the head or the HTTP header.

Duplicate cluster and canonical URLFour URL variants of the same category page form a cluster: an HTTP version, a version without www, one with the tracking parameter gclid and one with a sort parameter. Google picks https://www.example.com/dresses/ as the canonical URL. Below are the signals in order of strength according to Google: redirect strong, rel="canonical" strong, sitemap weak.DUPLICATE CLUSTERCANONICAL URLhttp://www.example.com/dresses/protocol variant (HTTP)https://example.com/dresses/host variant (no www)https://www.example.com/dresses/?gclid=ABCDtracking parameterhttps://www.example.com/dresses/?sort=pricesort function (parameter)Google pickshttps://www.example.com/dresses/usually shown in resultsand crawled mostregularlyWHAT INFLUENCES THE CHOICE, BY STRENGTH ACCORDING TO GOOGLE1redirect to the preferred URLstrong2rel="canonical" (HTML link or HTTP header)strong3URL listed in the sitemapweakGoogle also prefers HTTPS over HTTP.Schematic. The preference is a hint. Google may choose a different URL as canonical.Several signals pointing to the same URL are stronger together.
Google groups identical or very similar pages into a cluster and picks one canonical URL. According to Google, redirects and rel="canonical" are strong signals and the sitemap is a weak one. Your own preference remains a hint. Sources: Google Search Central: What is URL canonicalization; Google Search Central: How to specify a canonical URL with rel="canonical" and other methods.

Example

A partner page republishes an article and sets:

HTML
<link rel="canonical" href="https://www.example.com/blog/article/">

Common mistakes

  • The target is an unrelated page, such as the home page of the other domain.
  • The canonical was added by someone else and nobody on your side knows about it. Check the source of pages that suddenly lose rankings.

Sources

  1. Fix canonicalization issues – Google Search Centraldevelopers.google.com
  2. How to specify a canonical URL with rel="canonical" and other methods – Google Search Centraldevelopers.google.com

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