What is a canonical tag?

What is a canonical tag?

The canonical tag is all about duplicate content and preferred content. It is a rather unusual word, but etymologically speaking, it is appropriate. It is derived from canon, which originally referred to biblical or secular rules and laws, a standard for judgement. Later it was used to refer to the works of a writer that had been accepted as authentic.

This last meaning is where it relates to the Internet, SEO and search engines; helping search engines identify which is the original page in relation to duplicated content.

There are many legitimate reasons for duplicate content, particularly when it comes to system generated URLs. These include:

Multiple URLs – particularly on eCommerce sites where URLs are created through filter options for price, colour, rating, etc.

Session ID URLs – automatically generated by your system. The same applies to tracking URLs, breadcrumb links, printer friendly versions, and permalinks in certain CMS.

HTTP, HTTPS & WWW – search engines see Domain Names, Web Hosting, and Free Domain Services, Domain Names, Web Hosting, and Free Domain Services and Domain Names, Web Hosting, and Free as distinct pages, and will crawl (and possibly index) them as such.

Case – users, and most browsers, treat upper and lower case the same, with the two largely interchangeable. The same is not necessarily true for search engines, so if your website mixes up case in filenames and folder structure, you need to use the canonical tag.