Canonical link elements are a fix for certain duplicate content issues
A canonical webpage is essentially the preferred version of a number of pages that display the same or very similar content. This can happen in a number of circumstances – product pages on an e-commerce website display much the same content but have different URLs; instances where both www. and non-www. variations of pages exist or even a homepage that finishes in .com.au or .com.au/index.html or .com.au/index.php.
Other issues can arise where you create landing pages for different audiences with very similar content or Google Analytics tracking codes can result in numerous URLs displaying the same content, which Google may then interpret as duplicate content.
It’s important to address this to ensure variations of your page are not considered duplicate content and to ensure that Google knows which page you want indexed for the content displayed. If you have the same page displaying on multiple URLs and you don’t address it, Google will select one page to index and it may not be the variation that you are using in your linking strategy.
The best way to ensure that this canonical link issue doesn’t arise is to address it at development stage with uniform URL formats. If you use a popular CMS platform like WordPress or Joomla, SEO friendly URLs are now the norm but that’s not applicable in many cases.
WordPress also has specific plug-ins for canonical link elements or deal with the canonical issue as part of a more comprehensive SEO plug-in.
Another thing to get right is ensuring the correct variation (www. or non-www.) of URL is highlighted to Google. This can be done in your Google Webmaster Tools account. This variation should be consistent with what is in your sitemap and Matt Cutts from Google has acknowledged that where a sitemap exists, they will lean towards what is in the sitemap as the canonical version of a link.
How does the canonical link element work?
The canonical link element works similar to a 301 (permanent) redirect in that it tells Google and other search engines where you want them to go. The element should be placed within the head tag of the pages that have multiple URLs as follows:
<link rel=”canonical” href=”http://www.nameofyourwebsite.com”>
Canonical link elements should not be relied on as a solution to a problem
It is recommended (by Google and most SEOs) that other avenues are pursued before using the canonical link tag so as to avoid issues such as chain linking or looping of canonicals (on large sites) over time. Implementing and maintaining a consistent linking policy will ensure that the correct versions of URLs are used, which ultimately avoids the problem of duplicate content caused by the same page displaying on multiple URLs.
While the canonical link element is a useful ‘fix’ for this problem, it’s not the best solution. Issues can arise on large sites, where two different webmasters working on the site specify different canonical URLs, which leaves it up to Google to determine the most relevant – effectively rendering both canonical elements useless.
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