SEO and Canonicalization

Canonicalization is the term Google uses to describe the process of picking the best URL when there are more than one for the same content. For example the typical website homepage i.e. http://www.sample.com normally has four possible URL’s.

Those four links would normally be …

http://www.sample.com
http://sample.com
http://www.sample.com/index.php (or whatever)
http://sample.com/index.php

So what we have are in essence four different web pages, each gathering their own backlinks, but each one most likely having the exact same content. As a website owner or SEO this is obviuosly not something you want. The search engines also don’t want 4 versions of every single page cluttering up their databases.

This is where canonicalization comes in. Canonicalization gives you the power to choose which version you want to have the search engines use. It’s important that you make this choice for the search engines though because if you don’t, they will. While Google and other search engines are pretty good about deciding which version is best, its still better for us to retain as much control as possible.

To do this we will be using the canonical tag.

This tag is used inside of the <head></head> tags and will indicate to search engines which of the versions you want them to link to. You are then free to begin building links to your preferred version without worrying that some site somewhere will add links to the wrong one that you won’t get credit for.

I know there are probibly a few more questions but instead of repeating information Google has provided here are the links.

Matt Cutts on “SEO advice: url canonicalization”
Google Webmaster Central Blog on “Specify your canonical“