Advanced SEO : Using canonical references

What is Canonicalization?
The process of picking a single site URL to be indexed by the search engines from a range of URL’s. This happens when there are duplicate versions of the same/similar content on one site. Think of it as a way of suggesting the ‘true’ URL for your page(s) to the search engines

Why would I have multiple versions of the same content?
This usually occurs when you have dynamic content delivered from a database or there is more than one URL for a single page (typically the homepage, but not necessarily).
An example of this is an ecommerce site that enables users to get the same or very similar results from different actions.
E.g. If you have the URL of a product catalogue listing page built up from a series queries or filters.

The search engines understand this stuff?
Yes, they more than understand it they use it as a signal for search engine optimization. Since early 2009 Google, Yahoo and Microsoft have all stated they support this way for website owners to say “hey search engines, all these pages may have different URL’s but they are actually the same”.

What is the issue with duplicate content?
Search engines think that everyone is trying to game them and submit duplicate content to push themselves up the organic rankings. Without this, when a search engine finds duplicate content, it won’t now if you have done this accidentally or on. This means that they may well display the URL you don’t want to display, miss pages you want to get index or even worse downgrade the value of the page(s) they find… affecting the rankings and potentially the entire optimization of your site.

So how do I do this on my site?
If you read articles about this on the web, you would think that it is as simple as embedding a link in the header of your HTML pages (See: http://en.wikipedia.org/wiki/Canonical_meta_tag )

In theory this tells search engines the preferred location of the page to index (the “canonical” location) instead of the one it has found.

In practice it is more complex to create and maintain a working Canonical structure within an eCommerce site, especially one that has an evolving product catalogue.