Randfish over at SEOmoz.org has a great write up including a Q&A session about the ‘canonical‘ tag that is used to tell Yahoo!, Live & Google that a URL should be treated as though it were a copy of another URL and that all of the link & content metrics the engines apply should technically flow back to that URL. Although similar to the way in which a 301 redirect works, there are some differences which Randfish explains:
- Whereas a 301 redirect re-points all traffic (bots and human visitors), the Canonical URL tag is just for engines, meaning you can still separately track visitors to the unique URL versions.
- A 301 is a much stronger signal that multiple pages have a single, canonical source. While the engines are certainly planning to support this new tag and trust the intent of site owners, there will be limitations. Content analysis and other algorithmic metrics will be applied to ensure that a site owner hasn’t mistakenly or manipulatively applied the tag, and we certainly expect to see mistaken use of the tag, resulting in the engines maintaining those separate URLs in their indices (meaning site owners would experience the same problems noted below).
- 301s carry cross-domain functionality, meaning you can redirect a page at domain1.com to domain2.com and carry over those search engine metrics. This is NOT THE CASE with the Canonical URL tag, which operates exclusively on a single root domain (it will carry over across subfolders and subdomains).
If you’re into SEO, you’ll definitely want to read up on the proper usage of this canonical tag.