Anirudh Lohia

Google, Yahoo and MSN have announced that they will support a new canonical tag to reduce duplicate content confusion on the web and make things easier for everyone.

How does it work?

This tag is a part of head section in the html document of your webpage. the same section where you find title attribute and meta tags. Just like the way you include CSS file in your html document, simple use a new rel parameter. Example

<link rel="canonical" href="http://www.example.com/" />

This tag would tell Google, Yahoo and MSN the page in question is a copy of www.example.com and all the link and other metrics that search engines apply should flow back to that URL.

Before we go more in details of this tag let analyze the problem a little closely.

How is duplicate content generated on a website?

Duplicate content is generated on a website if one page has multiple URL’s

For example:

1. http://www.example.com/
2. http://example.com/
3. http://www.example.com/index.html
4. http://example.com/index.html

Theoretically all the above mentioned pages can be different, but practically all of them 99.9% of times will generate the same content. And this can be a big problem for search engines because they have no way to find out:

1. Which version to include or exclude in there indexes
2. Whether to direct link juice, trust worthiness to one page or keep them separate
3. Which version to rank is SERPS

And when this happens website owners are likely to loose Search Engine ranking and traffic.

Although there are some ways through this can control them upstream, like,

1. Making sure that your content management system generates consistent URL’s
2. Always link consistently on your website
3. Use 301 or Permanent redirect
4. You can specify preferred URL in Google webmaster tools.

But even with the mentioned solutions, duplicate content remains a major issue for search engines and webmasters because:

1. It can be possible that you cannot generate 301 redirect. Specially if you are using a free host.
2. Cannot help how people link to you
3. You may have session id’s in your URL or if you are buying ads, some kind of tracking code.
4. Or if you are running a ecommerce website and items on a page can be sorted in ascending or descending order based on its price
5. Or you give an option of printing the webpage to your user though a printer friendly page.

To all these problems we have a possible solution now. the CANONICAL TAG

We have already discussed about the placement of the tag earlier in this article. Now lets take an example

Suppose you have two URL’s for the same page lets say

http://www.example.com/item1.html , and
http://example.com/print/item1.html

Now both these pages have same content with only difference being that the second one is printer friendly page without side menu and other irrelevant element and the first one is normal page as user would like to see a webpage in a browser.

And obviously the webmaster would want all the link juice, trust, content juice to flow to http://www.example.com/item1.html from http://example.com/print/item1.html

For this what we can do is add the new canonical tag in the head section of http://example.com/print/item1.html

<link rel="canonical" href="http://www.example.com/" />

All the three major search engines will treat it as the mini 301 redirect and flow all the juice to the main page. Although if they find some abuse they reserve the right to choose any one as the main URL.

You can use this tag for sub domains or https and http versions but cannot use this tag across domains.

Matt Cutts recommends:

1. To use absolute paths intead of relative paths in the tag, and
2. for general user like mom and dad, to wait for sometime before tweaking your code, because surely there will be some plugins coming out for this shortly in future

Search Engines actually have quite a bit of information on this. Here are the links

Google
http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

Yahoo
http://ysearchblog.com/2009/02/12/fighting-duplication-adding-more-arrows-to-your-quiver/

MSN
http://blogs.msdn.com/webmaster/archive/2009/02/12/partnering-to-help-solve-duplicate-content-issues.aspx

This entry was posted on Tuesday, March 24th, 2009 at 7:44 am and is filed under Home. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.