Canonical URL tag: duplicate content no more

On February’12 the “Big 3″ search engines (Google, Yahoo! and MSN/Live/BING) bkore big news: it was annonced supporting a Canonical URL tag to help webmasters manage duplicate content issues. A propose, now this link-tag is also supported by Ask.com.

Duplicate content has long been an issue for search engines and marketers alike. Search engines want to avoid cluttering results with copies of the same text while marketers and website owners often need several copies of the same text with different URLs on their sites for usability or other purposes.

Google wrote: “… 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”.

Examples of duplicate content on ecommerce sites:

• A product is listed under multiple categories, each with its own URL
• The search engine crawls a site and is issued a session ID. It indexes links with the session ID
• A blogger copies a product link with a session ID or navigation tracking parameter like
  http://www.example.com/shop/products.php?ItemId=chicken&ref=acc_glance_sw_ai_549_1_img and unwittingly pastes the link as-is in a blog post
• An affiliate link like http://www.example.com/shop/products.php?ItemId=chicken&affid=1234 gets crawled and indexed
• Content is duplicated across sub-domains or sub-folders like www.example.com/shop/ or shop.example.com
• The search engine crawls your print friendly version

We try to compare new method and classical 301 redirect.

• Whereas a 301 redirect re-points all traffic (bots and human visitors), the Canonical URL tag is just for engines, meaning you can still separately track visitors to the unique URL versions.
• A 301 is a much stronger signal that multiple pages have a single, canonical source.
• 301s carry cross-domain functionality, meaning you can redirect a page at domain1.com to domain2.com and carry over those search engine metrics. This is NOT THE CASE with the Canonical URL tag, which operates exclusively on a single root domain (it will carry over across subfolders and subdomains).

Google’s FAQ on “canonical url tag”

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 ?

Yes, relative paths are recognized as expected with the tag. Also, if you include a 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.

SEOmoz blog has prepared the training image, how it works:

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