Website 101: Avoid Duplicate Content Penalties Use the New Canonical Meta Tag

In Feb 2009 at SMX West Google, Yahoo and Microsoft announced a new HTML tag. The new rel=”canonical” tag is designed to help reduce duplicate pages by documenting the preferred URL for a group of very similar pages. Duplicate content is a major problem for Search Engines and searchers alike. Having duplicate content on your site can lead to penalties in search rankings.

What is a Canonical Page?
Definition: Canonical Page is a preferred (authoritative) page among a group of pages that have identical or almost identical content. See Examples below for multiple home pages and Content Management System scenarios.

Goals of the new tag
The aim is to provide webmasters with a method to use to solve duplicate content crawl errors. Thus you can now specify a preferred version of a set of similar pages and ensure that link popularity is consolidated in your preferred page.
What you can now do is to indicate your preferred URL to the crawlers using an HTML link tag. This will result in better search results and higher rankings!

The Link Tag
The tag allows webmasters to designate a preferred page among a set of identical pages. It is used inside a element and is specified as rel=”canonical” href=”http://yourpreferredpage”>
The tag is used like this:
Rules for Using the New Tag
1) It is a suggestion and is NOT a command for the crawlers.
2) You can use relative or absolute URLs in the href=”yourdomain.com” tag part
3) The domain in the href=”yourdomain.com” MUST be the same domain as your preferred (canonical) page
 
EXAMPLE 1 – Multiple Home Page References
For example the four URLs below all lead to the SAME PAGE (a very common scenario):
http://my_company_site.com
http://www.my_company_site.com
http://www.my_company_site.com /default.htm
http://www.my_company_site.com/default.aspx
Suppose you as webmaster choose to use the second link (http://www.my_company_site.com) as your preferred page. Thus if you had a choice you would prefer if your PagePank value and your SEO rankings would all be channeled into that one page.
You can now do just that using the rel=”canonical” tag in the other 3 pages!
For the example above, this would look like:
EXAMPLE 2 – Auto Generated Content from Content Management Systems or Online Store software
Besides the examples for identical Home Pages given above, the link tag should also be used for pages having URLs that differ slightly because of having extra parameters like: session ids, sort orders, link etc.
http://www. my_company_site.com/product.aspx?item= Canned-Yellow-Peaches
http://www. my_company_site.com/product.aspx?item= Canned-Yellow-Peaches&page=check-out.aspx&ID=1234456
http://www. my_company_site.com/product.aspx?item= Canned-Yellow-Peaches&OrderID=ABC123
The two links above would be a good place to use the following canonical tag:

Add the tag to any pages on your website that would produce the product info for the Canned Yellow Peaches when called from various places on the site, for example just before checkout or as a link from a shopping cart as opposed to when the product is simply displayed on its primary page.
 
References:
1) http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html
2) Google Webmasters/Site Owners Help http://www.google.com/support/webmasters/bin/answer.py?answer=139394&topic= 15262
3) Yahoo Search Blog: http://ysearchblog.com/2009/02/12/fighting-duplication-adding-more-arrows-to-your-quiver/
4) Live Search Webmaster Center Blog: http://blogs.msdn.com/webmaster/archive/2009/02/12/partnering-to-help-solve-duplicate-content-issues.aspx