Google, Yahoo!, and MSN have come together to form a new meta tag, called the canonical meta tag. What does the canonical meta tag do? Why should you use it?

What is the canonical meta tag?

A canonical meta tag is a meta tag that you can use to specify what URL a search engine should use for your page. This is a great feature if you have duplicate pages. It helps most if you are using $_GET variables (dynamic pages). Here’s an example:

Let’s pretend that you have a page at http://domain.com?p=17. The page displays post 17. When a user (or a bot) clicks to go to the next comment page, it becomes http://domain.com?p=17&c=2. The search engines will index these pages seperately. You can use the canonical meta tag to fix this. Here’s what it would look like:

<link rel="canonical" href="http://domain.com?p=17" />

If this tag appears on both of the pages (http://domain.com?p=17 and http://domain.com?p=17&c=2), both pages would be indexed as the same page, eliminating the duplicate content. Pretty cool. You can use the canonical meta tag in many situations. If you have a session id appended on some of your URLs, you can use it to eliminte duplicate content created because of that. It’s really a great tool, I’m glad that all of the major search engines added it.

In addition to eliminating duplicate content, the canonical meta tag will transfer PageRank and other similar properties. If your page http://domain.com?p=17&c=2 has a PageRank of 2, and then you send it’s canonical URL to http://domain.com?p=17, the PageRank will be transferred.

I’m pretty sure that all of the search engines have good ways of telling if you are cheating. They don’t use the canonical URL everytime. It’s just a suggestion for the search engine, you can’t force the search engine into choosing one URL. The cool part will be when the canonical meta tag is totally unecessary. Instead, the search engines determine what URL to use on thier own. It might be a little while until this happens (it’s basically artificial intelligence), but I’m sure that one day it will.

You can read the press releases by Google, Yahoo!, and Microsoft for more information.

Tags: Canonical Meta Tag, HTML Tags, Meta Tags, Search Engine Optimization