developers.google.com

Specify your canonical  |  Google Search Central Blog  |  Google for Developers

  • ️Thu Dec 17 2009

Specify your canonical

Stay organized with collections Save and categorize content based on your preferences.

Thursday, February 12, 2009

Carpe diem on any duplicate content worries: we now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.

Let's take our old example of a site selling Swedish fish. Imagine that your preferred version of the URL and its content looks like this: https://www.example.com/product.php?item=swedish-fish

However, users (and Googlebot) can access Swedish fish through multiple (not as simple) URLs. Even if the key information on these URLs is the same as your preferred version, they may show slight content variations due to things like sort parameters or category navigation: https://www.example.com/product.php?item=swedish-fish&category=gummy-candy

Or they have completely identical content, but with different URLs due to things such as a tracking parameters or a session ID: https://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

Now, you can simply add this <link /> tag to specify your preferred version: <link rel="canonical" href="https://www.example.com/product.php?item=swedish-fish" /> inside the <head> section of the duplicate content URLs:

https://www.example.com/product.php?item=swedish-fish&category=gummy-candy
https://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

and Google will understand that the duplicates all refer to the canonical URL: https://www.example.com/product.php?item=swedish-fish. Additional URL properties, like PageRank and related signals, are transferred as well.

This standard can be adopted by any search engine when crawling and indexing your site.

Of course you may have more questions. Joachim Kupke, an engineer from our Indexing Team, is here to provide us with the answers:

Is rel="canonical" a hint or a command?
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, for example, 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.

Sounds great—can I see a live example?
Yes, wikia.com helped us as a trusted tester. For example, you'll notice that the source code on the URL https://starwars.wikia.com/wiki/Nelvana_Limited specifies its rel="canonical" as: https://starwars.wikia.com/wiki/Nelvana.

The two URLs are nearly identical to each other, except that Nelvana_Limited, the first URL, contains a brief message near its heading. It's a good example of using this feature. With rel="canonical", properties of the two URLs are consolidated in our index and search results display wikia.com's intended version.

You can ask additional questions in our comments below. And if you're unable to implement a canonical designation link, no worries; we'll still do our best to select a preferred version of your duplicate content URLs, and transfer linking properties, just as we did before.

For more information, please see our Help Center articles on canonicalization and rel=canonical.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Need to tell us more? [[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],[],[[["Websites can now specify their preferred version of a URL for pages with duplicate or very similar content using the `rel=\"canonical\"` link element."],["This helps search engines consolidate properties like link popularity to the preferred version and have more control over the URL displayed in search results."],["The `rel=\"canonical\"` tag is a hint, considered alongside other signals, to determine the most relevant page to show."],["Slight content variations between the canonical and duplicate pages are acceptable, and the tag can be used across subdomains but not across different domains."],["While implementing this is recommended, search engines will still attempt to select a preferred version and transfer link properties even without the tag."]]],["The content introduces a method to manage duplicate content URLs. By adding a `\u003clink rel=\"canonical\"\u003e` tag within the `\u003chead\u003e` section of duplicate pages, website owners can specify their preferred URL. This helps consolidate link popularity and signals towards the chosen URL, ensuring it's favored in search results. The feature handles slight content variations and is a \"hint\" to search engines, not a command. The method works within the same domain or subdomains but not across different domains.\n"]]