On February 19, Google announced a new tag that specifies the standard version of a URL to help avoid duplicate content issues with search engines. The new tag specifies the canonical, or standard version of a URL. MSN and Yahoo also announced that they will recognize the canonical tags.
Google announced the canonical tag in their Webmaster Central Blog. This new tag solves a range of SEO issues that have sometimes been difficult to overcome.
The problem is that sometimes several URLs can represent a single web page. This can create duplicate content issues that result in pages that do not rank well, plus it splits the inheritance factors, such as Google PageRank and reinforcement of keywords themes in hyperlink text. Ideally, you want to have all of a site’s inheritance factors and “link juice” focused on a single version of a URL. Standardizing a site’s URLs has been a basic tenant of SEO for several years–and one that is sometimes difficult to deal with. This new tag makes the task much simpler.
Here are some classic variations of the home page URL.
With the canonical tag, you can specify your preferred version of any page URL. In the case of the home page, it should be http://www.mydomain.com/ or http://mydomain.com/, depending upon which version you have chosen to be your standard domain name.
The canonical tag is a link tag that should be placed in the head section of a web page.
<link rel="canonical" href="http://www.mydomain.com/" />
The canonical tag for each page will be different, depending upon how you wish to specify the standard version of a page URL. According to the specification set by the search engines, the canonical tag can use an absolute or a relative URL path in the tag.
An absolute path looks like this:
<link rel="canonical" href="http://www.mydomain.com/my-article/" /> or <link rel="canonical" href="http://www.mydomain.com/page.php?page_id=66" />
A relative path looks like this:
<link rel="canonical" href="/my-article/" /> or <link rel="canonical" href="page.php?page_id=66" />
This particularly important for sites using Microsoft servers that do not have a method to set a standard version of a domain name, and for sites that may be experiencing problems with session IDs or reference parameters in the URLs that are being picked up by search engines.
If your web site uses static pages, the canonical tag wil have to be manually placed in the head section of a web page’s code. if you use WordPress, several plugins, including the popular all-in-one-seo-pack, will automatically place the correct canonical tag in the head section if you have that feature activated. For dynamic sites, the code may have to be written to add this tag to the code.
It can be worthwhile to add the canonical tag to a web site because it avoids a number of potential issues, many of which can have a negative impact upon your search engine rankings.
Matt Cutts, one of Google’s quality engineers, posted some useful tips about the canonical tag, which he refers to as the canonical link element in his blog. The blog posting has many useful links to other references regarding this new search engine optimization tool.