YNOT – More than a year ago, the three big search engines — Google, Yahoo and Bing — reached a revolutionary agreement under which use of a common tag would help all of them to improve the quality of search results.

That tag is the canonical tag.

A couple of questions ensued. Would the engines really using the tag? How important would the canonical tag become for SEO results? The answer to the first is “yes.” This article attempts to answer the second.

Let’s start by defining what the canonical tag does: It offers a method to decrease duplicate content.

There are many reasons duplicate content may appear within website. The most common reason is that several URLs point to the same webpage. Why might that happen?

• Analytics tracking codes have been added to URLs (e.g., Omniture).
• Pages generated from a search box offer different options, but at the end show the same product.
• Affiliate IDs are appended to URLs.

…and so on.

So, for example, let’s say you have an e-commerce site selling adult content. The canonical URL is [URL]http://www.example.com/adult-content[/URL], which may be reachable through:

• Search box: [URL]http://www.example.com/adult-content?=01?cat=main[/URL]
• Affiliate ID: [URL]http://www.example.com/adult-content?id=001[/URL]
• Analytics tracking: [URL]http://www.example.com/adult-content-Y3434030Y[/URL]
• Canonical: [URL]http://www.example.com/adult-content[/URL]

Each of the different URLs displays the identical page to an end-user.

Now, how can the search engines understand which is the real page? If you don’t specify, crawlers will waste time crawling the same content with different URLs and your external linking efforts could be diluted.

That’s why the search engines came up with the canonical tag. This is how works:

Specify the canonical version using a tag in the header section of the page as follows:

That’s it!

The canonical tag functions as a 301 redirect for all URLs that display the page with the tag, but it redirects just search engines, not users. The tag is invisible to users.

Clearer now?

When the search engines launched the new tag, many SEO professionals were a bit skeptical. We saw it as a huge revolution for SEO in theory, but we were not sure it would work in practice because the tag was offered as a suggestion, not a mandatory component.

More than one year after launch, the canonical tag is working, sometimes in sneaky ways. (If you’re an affiliate who deploys white-label sites in your arsenal, check to ensure any canonical tags embedded in their code display your site, not a URL belonging to the white-label provider. Otherwise, your SEO efforts will benefit the provider, not you.) On the positive side, though, we have used canonical tags on thousands of pages to remove duplicate content and phantom affiliate pages from search engine results (SERPs), and the tags work perfectly.

Search forms, quite common across all kinds of sites, are one of the biggest culprits in confusing SERPs, and most of the time webmasters don’t realize how pages generated by search forms can mess up an otherwise carefully crafted SEO plan. By using the canonical tag, webmasters can “normalize” any automatically generated URLs.

Canonical tags have worked as the search engines promised, making life easier for the engines, webmasters and end-users.

Even better: Google now gives more value to the canonical tag as opposed to the 301 redirect, especially redirects between domains.

One final caveat: Double check all canonicals to ensure the tag on each page gives the information you meant to convey. Tagging an important page with the wrong canonical URL is tantamount to telling the search engines “I don’t want this page anymore. Please redirect it to somewhere else.” That could be an expensive mistake.

This column was contributed to YNOT by the SEO staff at RIVCash.com.