What is canonical Tag ?

The canonical tag is a page level meta tag that is placed in the HTML header of a webpage. It tells the search engines which URL is the canonical  version of the page being displayed. It's purpose is to keep duplicate content out of the search engine index while consolidating your page’s strength into one ‘canonical’ page.

How is canonical tag used?

The canonical tag is a relatively quick solution to resolve duplicate content. If your website generates and displays the same (or very similar) content on multiple URLs, the canonical tag could be used to bucket them together and assign one master (canonical) version. Lets look at a list of common duplicate content URLs.

• http://example.com/quality-wrenches.htm (the main page)
• http://www.example.com/quality-wrenches.htm (oops! all pages also resolve with the www sub-domain)
• http://example.com/quality-wrenches.htm?ref=crazy-blog-lady (this looks like a way to track referral sources)
• http://example.com/quality-wrenches.htm?sort=price (how users view the products by lowest to highest price)
• http://example.com/quality-wrenches.htm/print (the ad-free and graphic light print version)

A canonical tag that references the main page, http://example.com/quality-wrenches.htm, could be placed in the header of all of the above pages.

How is canonical tag implemented?

The canonical tag is part of the HTML header on a webpage. This is the same place where we put other fun SEO stuff like the title tag, meta description tag and the robots tag. The code, as in my example above, would look like this.

<link rel="canonical" href="http://example.com/quality-wrenches.htm"/>