Google, Yahoo and Live search have introduced a great new way to serve linkers different content than other visitors or search engines. Thanks to the rel=canonical tag the search engines are now supporting they will need to implement many new spam detection methods as well.
If you do not yet know about the tag please read Rand Fishkin’s rel=canonical post or the information from Google on the new canonical tag.
How does it work?
With the new canonical tag you indicate that there might be more than one URL within your website presenting this specific information and if search engines want to choose between those when presenting it to visitors, you would prefer the given URL. I for instance use wordpress to rewrite to nice URLs but this post is available under http://www.vdgraaf.info/?p=120 as well.
If I would place <link rel="canonical" href="http://www.vdgraaf.info/canonical-tag-creates-new-legal-cloaking-possibilities.html" /> in the head of this page, Google would only show that one to visitors. But just like a 301-redirect Google would attribute all linkjuice to the remaining page.
The canonical tag is intended for sites that cannot do a proper 301 redirect because of several reasons. For instance if there still are slight differences between the pages under different URLs, but too little for Google to see them as unique. The same article might be available under different categories, an affilliate ID in the URL just changes some form values or you use almost the same text for the French speaking population of Canada as you do for people in France. I can think of many more, but all these legitimate reasons allow for slight changes to the page served. And it is logical that search engines now allow you to indicate which version to choose.
One page, different audiences
Your website is created for several audiences and in the ideal situation you could change nuances depending on the visitor type. The most important audiences we want to differentiate right now are “linkers”, “navigators” and “searchers”.
- People linking to your website
A clean webpage without much commercial intent is far more likely to recieve inbound links than a page with a lot of branding and call-to-actions. A page that shows your good side (for instance with a reference to your altruism to good causes, or a reference to the specific link partner) will give you a far higher success rate on your link building effort. Use a URL like http://www.vdgraaf.info/i-am-a-good-boy.html for them. - People navigating from within your website
People that navigate through your website follow a certain path and you can offer more specific content depending on where they came from. http://www.vdgraaf.info/so-you-clicked-the-banner.html would be good for this audience. - People from search engines
A good landing page for search engines gives the answer to what you searched for and shows a clear call-to-action to the most logical next step. Besides changing the appearance of the page that you would like search engines to serve, you could also use slightly different code to be more search engine friendly. You should make this version the canonical one by adding<link rel="canonical" href="http://www.vdgraaf.info/this-one-is-for-searchers.html" />to the head section of all your versions.
Does the tag give real cloaking abilities
It is logical that Google will or already has implemented a check on simularity between different versions. Because the tag is onpage and not in an external file like the robots.txt, the page has to be spidered first. It is a small extra step to do some text calculation as well. The canonical instruction will probably be followed when the page would normally trigger duplicate content filter. It will however assign all accumulated links to that one remaining occurrence.
So use this tag with care, but with the same content you can still create many different versions. Have fun!