Duplicate content, canonical link element (tag) instructions on use

This is used where there are multiple versions of the same page (ie the same content) on a site.

Say the following three URLs all generate the same case study.

http://www.mydomain.com/houses/casestudy.php

http://www.mydomain.com/bungalows/casestudy.php

http://www.mydomain.com/flats/casestudy.php

(If each page has its own banner saying ‘Houses’, Bungalows’ or ‘Flats’ with a short intro para, then they are still the same page as far as duplicate content is concerned.)

The solution is to ensure only one of the URLs is indexed by Google and this is achieved using the canonical link element.

Select the ‘right’ URL – ideally the simplest if that is a relevant criterion – and then add the following tag to the head section of all duplicate versions of the page.

So if the houses version is one we want indexed the element would look like

<link rel=”canonical” href=” http://www.mydomain.com/houses/casestudy.php”/>

and should be placed in the head section of all duplicate pages, in this case

http://www.mydomain.com/bungalows/casestudy.php

http://www.mydomain.com/flats/casestudy.php

Google should then ignore these two versions of the page.

The canonical link element is only advisory but Google says it will do its best to implement it, and will only over-ride it if it finds abuse (hard to envisage) or if a site accidentally misuses it, eg by creating a loop (if you have two pages and you put a link element in the headers of each saying the other is the right version).

The canonical link element is designed to act in the same way as a 301 in that any link juice pointing to a duplicate page will be transferred to the right page.

Yahoo! and Bing plan to support the canonical link element but aren’t thought to have done so yet.

Google advises the avoidance of duplicate pages wherever possible and the use of the canonical link element as a last resort – but a very useful one.

Nomenclature

Technically it is a link element but no-one is going to get excited if it is called a link tag, analogous to a meta tag.

The right page is the canonical version and the duplicates are the non-canonical versions.

For further information see Google’s support notes at http://www.google.com/support/webmasters/bin/answer.py?answer=139394

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