http://www.example.com/product.php?item=swedish-fish

http://www.example.com/product.php?item=swedish-fish&category=gummy-candy

http://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

<link>

<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish" />

<head>

http://www.example.com/product.php?item=swedish-fish&category=gummy-candy

http://www.example.com/product.php?item=swedish-fish&trackingid=1234&sessionid=5678

http://www.example.com/product.php?item=swedish-fish

<link rel="canonical" href="product.php?item=swedish-fish" />

<link>

<base>

www.example.com

example.com

help.example.com

example.com

example-widgets.com

http://starwars.wikia.com/wiki/Nelvana_Limited

http://starwars.wikia.com/wiki/Nelvana

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: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:Or they have completely identical content, but with different URLs due to things such as a tracking parameters or a session ID:Now, you can simply add thistag to specify your preferred version:inside thesection of the duplicate content URLs:and Google will understand that the duplicates all refer to the canonical URL:. 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: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.Yes, relative paths are recognized as expected with thetag. Also, if you include alink in your document, relative paths will resolve according to the base URL.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.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.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.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.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.Update on 12/17/2009: The answer is yes! We now support a cross-domain rel="canonical" link element.Previous answer below: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 suggestvs.vs., but notvs.Yes, wikia.com helped us as a trusted tester. For example, you'll notice that the source code on the URLspecifies its rel="canonical" as: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.Feel free to 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 : this link-tag is currently also supported by Ask.com Microsoft Live Search and Yahoo! for more information, please see our Help Center articles on canonicalization and rel=canonical