Canonical URL Links to Combat Duplicate Content

Google, Yahoo, and MSN have just announced a new tag to combat duplicate content. They now support Canonical URL Links, which is a format that allows you to publicly specify your preferred version of a URL.

Canonical URL Links will help if your site has content that is:

• Identical or vastly similar
• 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 link.

I’ll use the Google Webmaster Blog’s Canonical URL Example below…

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 this
tag to specify your preferred version:

Place it inside the section of the duplicate content URLs:

Search engines will understand that the duplicates all refer to the canonical URL: ‘http://www.example.com/product.php?item=swedish-fish’.

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.

Joost de Valk wrote a great article on canonical urls and also created a very nice WordPress Plugin for Canonical URLs.