Using the canonical tag to deal with duplicate content
Whenever we’re doing search engine optimisation work on client sites one of the biggest issues we face is duplicate content pages. Many content management systems create several versions of a page with the same content, as there are many instances when these pages are required. Duplicate content pages can however cause problems with the search engines as they are unsure which version of the page to list in their index.
Consider for example a category page on an e-commerce site, in this case showing shoes. This might have a URL that looks something like this:
You are able to order the shoes category page by name, price or season, this however generates 3 copies of the page with the same content but slightly different URLs:
As discussed this now causes problems as the search engines see four pages when we only want them to list one. A secondary issue is that each of the pages will have some PageRank; ideally we want to concentrate all this PageRank on a single page to achieve a higher ranking.
The normal procedure with duplicate content pages is to redirect all the copies to the original page using a 301 (permanent) redirect. The problem here is that in the situation above this isn’t possible as the user needs to be able to view all versions of the page.
The Better Way – Canonical Tag
There is a tag which will tell the search engines to ignore the duplicate pages and pass all ranking value down to the original page.
<link rel="canonical" href="http://www.example.com/category/shoes " />
The URL you need to use is the URL of the original page that you’d like to appear in the search engines.
There are a few points worth noting if you intend to follow this approach:
- It is recommended that you use absolute URLs in the canonical tag to avoid the chance of errors.
- The canonical tag will only work with pages that are similar; some changes are allowed such a different sort order for the content on the page. Too much change and the search engines will ignore the tag.
- The canonical tag can only be used across multiple domains, for more information read Handling legitimate cross-domain content duplication
- The tag is currently supported by Google, Yahoo! & Bing