Why you need a canonical tag?

"Duplicity" is the scariest word for website owners.

Duplicity can be of two types:

1. Internal duplicity: When the content within the website is duplicate or there are multiple versions of URLs existing (specially in case of e-commerce websites) 
2. Let's call it external duplicity: When you are lazy to write more and accidentally writes the website's content onto some other websites (like, company profile in linkedIn) or someone was lazy and copied your website's content onto his.

It was duplicate content which made Google to conceive Panda update. Panda target websites with poor, low quality content or duplicate content.

By now, many of you must have found a tool to check duplicate content, but for those who are searching, copyscape.com is the fantastic tool to check the duplicate content. Copyscape, shows the various websites having similar content as that of your website. 

While copyscape helps is tracing external content, how can you trace internal duplicity? Well, webmaster tools are the solution. Webmaster will not show you the duplicate content, it will show the duplicate meta tags (meta title and meta description). These details are available in "HTML Improvements" section. 

Now that you have tracked the internal duplicate content, what is the next step? Check why these pages are duplicate. You may notice there are pages where you tend to have same content over different pages. Such issues are faced by e-commerce websites. 

Let us understand with an example, say you sell shirts online, you decide to show 10 shirts on one page and make such 8 pages. For SEO purpose you add a small content in the end of the page. Now when crawlers crawl the website, they'll notice that the URL is changing from, http://myshirtsexample/shirts/page1 to http://myshirtsexample/shirts/page2 and so on up to 8 pages, but the content is same on all of the 8 pages, it raises the trigger, considering the content as duplicate. But in actual, the content is not duplicate and it is practically impossible to write content of 8 pages (or more pages). 

Let us take one more example, which is a general problem in every website. At times, the home page tends to have many versions of URL. www.mywebsite.com , www.mywebsite.com/index.html, mywebsite.com etc. In such cases too, crawlers will treat each version as different websites containing same content.

Now with the two examples above you have managed to track types of duplicity that are possible. Some are easily removed and for some other is the "canonical tag".

With help of canonical tag, you can notify Google that the content on these pages are not duplicate. The canonical tag looks like,

 <link rel="canonical" href="www.mywebsite.com" />

Let's understand, I'll put this tag in the head section of, www.mywebsite.com/index.html and mywebsite.com. This notifies Google that my original website is "www.mywebsite.com". 

For the shirts example, the canonical tag that Google has suggested is rel=”next” and rel=”prev”. Now about using it.

On the webpage http://myshirtsexample/shirts/page1, add the following canonical tag:

<link rel="next" href="http://myshirtsexample/shirts/page2" />

On second webpage http://myshirtsexample/shirts/page2, add following canonical tag:

 <link rel="prev" href="http://myshirtsexample/shirts/page1" />

<link rel="next" href="http://myshirtsexample/shirts/page3" />

Now there's may be a chance when you are changing the domain and some how 301 redirect is not possible, for such cases, Google supports, cross-domain rel="canonical" link element. Choose wisely.

Note:  

1. While Google can understand numbers of canonical tags on single website, Bing usually advice not to use too many canonical tags.
2. Choose canonical link wisely, the link that is put along it is the main link.
3. While using next, prev canonical use, next tag on first page, prev on last page and next and prev together on second-to-second last page.
4. Before using cross-domain canonical tag, try 301 redirects but if somehow it is not at all possible then go for cross domain canonical tag.