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Define Canonical URLs for search engines

  • 17-02-2009 6:28pm
    #1
    aidan_walsh
    Closed Accounts Posts: 17,208 ✭✭✭✭


    In case it hadn't reached you, Google, Yahoo and Microsoft all announced support for a new link tag over the weekend to specify a canonical URL in the case where there may be duplicates.

    The tag takes the format <link rel=”canonical” href=”http://www.example.com/products” />

    Each of the blogs break down the details, but here is a little Q&A from the Google blog.
    Is rel="canonical" a hint or a directive?
    It's a hint that we honor strongly. We'll take your preference into account, in conjunction with other signals, when calculating the most relevant page to display in search results.

    Can I use a relative path to specify the canonical, such as <link rel="canonical" href="product.php?item=swedish-fish" />?
    Yes, relative paths are recognized as expected with the <link> tag. Also, if you include a <base> link in your document, relative paths will resolve according to the base URL.

    Is it okay if the canonical is not an exact duplicate of the content?
    We allow slight differences, e.g., in the sort order of a table of products. We also recognize that we may crawl the canonical and the duplicate pages at different points in time, so we may occasionally see different versions of your content. All of that is okay with us.

    What if the rel="canonical" returns a 404?
    We'll continue to index your content and use a heuristic to find a canonical, but we recommend that you specify existent URLs as canonicals.

    What if the rel="canonical" hasn't yet been indexed?
    Like all public content on the web, we strive to discover and crawl a designated canonical URL quickly. As soon as we index it, we'll immediately reconsider the rel="canonical" hint.

    Can rel="canonical" be a redirect?
    Yes, you can specify a URL that redirects as a canonical URL. Google will then process the redirect as usual and try to index it.

    What if I have contradictory rel="canonical" designations?
    Our algorithm is lenient: We can follow canonical chains, but we strongly recommend that you update links to point to a single canonical page to ensure optimal canonicalization results.

    Can this link tag be used to suggest a canonical URL on a completely different domain?
    No. To migrate to a completely different domain, permanent (301) redirects are more appropriate. Google currently will take canonicalization suggestions into account across subdomains (or within a domain), but not across domains. So site owners can suggest www.example.com vs. example.com vs. help.example.com, but not example.com vs. example-widgets.com.


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