The Canonical Link Element (also referred to as the “Canonical tag”) has been around for over two years now and has received a bit of attention lately as more and more websites utilize this powerful link to respond to a variety of duplicate content issues. But there are clear instances when the Canonical tag should be used and times when it should be avoided.
I like to think of the Canonical Link Element as the “spare wheel” of website architecture. It works in an emergency but it comes with performance issues. But if you have ever had a flat tire, hooking up the spare and driving slowly is far better than being dead in the water.
Duplicate content occurs when multiple URLs return the exact same content. Examples include www vs non-www domains, adding /index.html to the home page URL, capitalizing URLs, session Id’s, tracking code from marketing campaigns and many, many others. Search engines really frown upon duplicate content – it costs them database resources, causes confusion about the validity of URLS and can fragment pageRank. As a result, websites can suffer in search engine rankings if duplicate content infractions are substantial.
Back in February of 2009, the big 3 search engines (Google, Yahoo and Microsoft – but let’s be current and just say Google and Bing) came together to recognize the Canonical link element as a way to deal with duplicate content problems. Jill Whalen famously stated that “developers keep SEOs in business” and there are many ways sub optimal coding can cause duplicate content. But beyond the myriad of developer and coding issues that can cause duplication, the problem is exacerbated when Marketing teams append tracking parameters to URLs in advertising campaigns.
One piece of advice given by Matt Cutts regarding the use of the Canonical link element was that it is “far better” to avoid duplicate URLs in the first place and “exhaust alternatives” before turning to the Canonical element. For instance, handling the “www” vs. “non-www” URL duplication is best executed on the server if possible. Creating a consistent internal linking structure and a consistent XML site map should be enforced by your website’s code, not the Canonical link element.
The situations that are often out of the hands of developers should constitute the limited scope where the Canonical link should be used. For example, very difficult duplicate content management issues can be caused by tracking code parameters, inconsistent links from external domains and restrictive CMS systems that create unique URLs for non-unique pages, such as sort order.
Although it serves a good purpose, the use of the Canonical tag should be strategic and used only when necessary. It is a crutch with a limited scope of uses.