rel=canonical tag | Canonicalization

What is the rel=canonical tag?

The rel=canonical tag is a rel attribute usually placed on duplicate content and tells search engines that the content is a copy and where the original content is based. The rel=canonical tag is supported by all 3 major search engines and was announced as by Google, Yahoo! and Microsoft in February 2009. Rand Fishkin of Moz described it as the “most important advancement in SEO practices since sitemaps”. In fact, Facebook and Twitter also recognise rel=canonical.

Unlike a 301 redirect, the rel=canonical tag in now way affects the user’s experience. Whilst a 301 redirect will only change the URL and therefore may actually be fairly imperceptible for the user, a rel=canonical tag is only seen by robots, more specifically by search engines. From a search engine’s perspective it means

It would also appear that like 301 redirects, when Google recognises a rel=canonical tag it passes the majority of SEO metrics such as link metrics although, again like a 301, there may be a small element of ‘leakage’.

What is a Canonical URL?

SEO Considerations for rel=canonical tag

rel=canonical tags are often used as failsafe for 301 redirects – webmasters will 301 redirect duplicate content to the original URL and also pace a rel=canonical tag on the page to ensure that the search engines understand. Regardless, there are a number of SEO considerations for canonicalization:

You want to ensure that the target URL of your rel=canonical tag is a viable page. That is to say that it exists as a page (does not return a 404 error) or has not had a robots no index meta tag placed on it (and therefore invisible to search engines anyway!).
Do not apply more than one rel=canonical to any one page. This may seem obvious but stranger things have happened! When there is more than one rel=canonical tag on a page, search engines ignore them all.
Ensure that the correct page is designated as the target, i.e has very strong similarities to the ‘duplicate’.
Do not rel=canonical paginated pages back to ‘page 1’. Instead use rel+prev/next or use the view all option (if it exists) as the canonical.
You do not have to put rel=canonical tag on the original document (i.e the one that all the other duplicates specify as their target). Some webmasters do so as a way of automating the process and being able to apply the tags more quickly.

Technical Implementation

Technical Implementation
The rel=canonical tag is placed between the <head><head> of a web page as a new rel parameter. It should not be included in the <body> of the webpage as it does not count.

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