A canonical tag is simply a short piece of code that is placed on a webpage to let the search engines know which nearly identical piece of content is the original and which is the duplicate. By identifying the original, you can avoid any problems with duplicate content and ensure that only the original content gets indexed and receives the link credit.

Some common situations where you might use a canonical tag include

• A mobile website that is set up a sub-domain or sub-directory of your main website. A canonical tag can help the search engine robot understand how these similar pieces of content are related and thus do not constitute duplicate content. You will also want to add a ‘rel alternate’ tag on your desktop version that will point back to your mobile content.
• A blog which uses tags to help organize individual blog posts. If multiple posts are tagged with more than one category label, they might all appear in each of those categories and be considered duplicate content. A ‘rel canonical’ tag will identify these as a category view and not as an intentional duplicate content issue.
• If your website presents content broken down into separate categories where one category might contain the same content as another, you can place a canonical tag on each category to avoid any duplicate content issues.

The Problem with Duplicate Content
Search engines are all about providing the best possible user experience, which means that they will usually not display duplicate content. Without canonical tags to guide them, the search engine robots will have to make their own best guess as to which piece of nearly identical content is the correct one to index.

Other options for server redirection include 301 and 302 redirects.

• The 301 redirect is used to indicate that a particular web page has been permanently moved to another URL. A 301 passes between 90 and 99 percent of the original link juice or ranking power to the new destination. In essence a 301 redirect tells both the search engines and your website visitors that your original page has been permanently replaced.
• A 302 redirect is a temporary solution that passes very little if any link juice.

A ‘rel=canonical’ tag passes virtually the same amount of link juice as a 301 redirect and is usually a much simpler and quicker option to implement.

On the Plus Side, Canonical Tags

• Provide webmasters with a high degree of control in determining which version of a given page will appear in the search results.
• Can be easily implemented without having to access the site root.
• Preserve the majority of the link value.
• Are a clean fix. When properly implemented, canonical tags effectively mask the duplicate content from the search results.

On the Minus Side, Canonical Tags
Are frequently ignored by the search engines, especially if they have been incorrectly implemented. Can be difficult to implement correctly. The majority of websites that use canonical tags have them incorrectly configured, which forces the search engine robot to choose what it thinks you are trying to accomplish.