Quick Guides: The Canonical Tag

What is the Canonical Tag?

The Canonical Tag is a HTML tag that helps search engines to deal with pages that contain duplicate content. The tag is mainly used on pages that are slight variations of an original page, such as landing pages, printer friendly pages, gateway pages etc.

How is the Canonical Tag used?

The Canonical Tag is used to tell search engines that the URL containing the tag is a duplicate of another URL. Meaning that the rankings, link juice etc on that page should be given back to the other (and original) URL. Its like saying that all pages with a Canonical Tag on linking back to the same main page should be treated as one big page.

There are a number of different reasons for using the Canonical Tag; a good example to use is ‘printer friendly’ pages. If a webpage is duplicated and simplified into a printer friendly webpage, search engines will want to try and crawl that content.

This means that there are two pages with exactly the same body of content and obviously you don’t want this as it can cause duplicate content issues for your site. The original webpage is the page is the page that was initially designed for web viewing, so that’s the one that should be indexed by search engines.

In order for this to happen the printer friendly page needs to contain a canonical tag referencing the original page.

What does the Canonical Tag look like?

In the above situation, the Canonical Tag will appear on the printer friendly page like this:

<link rel=”canonical” href=http://www.originalpage.com />

How do search engines look at the Canonical Tag?

The best example of how search engines look at the Canonical Tag can be taken from The Art of SEO (Spenser, Enge, Fishkin, and Stricchiola), and looks like this:

When a search engine is faced with a page that has a Canonical Tag referencing another site it will treat the page almost as an extension of the referenced site.

How does the Canonical Tag differ from a 301 Redirect?

The Canonical tag differs from a 301 redirect in that it is only used for search engines – human’s will still be able to view each page involved in the canonical cycle.

A 301 redirect will point humans to a different page, effectively making the URL with duplicate content (the printer friendly page) redundant. When that URL is entered into a browser, it will simply point to a different page.