What is meant by Canonical URL and How to specify a canonical with rel="canonical"?

Canonical URLs are the preferred URLs for a webpage, which are designated as the "canonical" version of that page, and are used to eliminate duplicate content issues in search engine optimization (SEO).

Duplicate content occurs when the same content is accessible through multiple URLs, which can create confusion for search engines and may result in a lower ranking for the affected pages. To avoid this, website owners can designate a canonical URL that will be considered the primary version of the content, and will be used by search engines as the reference point for indexing and ranking.

For example, if a webpage can be accessed through different URLs such as 

http://example.com/page1 and 

http://www.example.com/page1, 

the website owner can specify one of these URLs as the canonical version. This tells search engines that the specified URL is the preferred URL to index, and any other version of the page should be considered a duplicate.

Canonical URLs are typically specified using the HTML "link rel=canonical" tag in the header of the page. This tag tells search engines which URL is the canonical version of the content, and should be used for indexing and ranking purposes.

Canonical URL is the preferred URL for a webpage that is chosen by website owners to indicate the primary version of a particular content. It is used to prevent duplicate content issues, which can harm the website's search engine rankings. By designating a canonical URL, webmasters can indicate which version of a page should be indexed by search engines, which can help to avoid confusion and maintain the website's SEO performance.

To specify a canonical URL, you can use the rel="canonical" link element in the HTML code of the page. This element tells search engines which version of a page is the preferred version, and should be indexed and ranked accordingly.

For example, if a webpage has multiple versions, such as: 

 http://example.com/page1 

 http://example.com/page1/?source=social 

 http://www.example.com/page1

You can add a canonical link element to the head section of the HTML code of each page to specify the preferred URL. For instance, if you want to set "http://example.com/page1" as the canonical URL for the above versions, you can add the following line of code to the head section of each page:

link href="http://example.com/page1" rel="canonical"

By specifying the canonical URL, you can ensure that search engines consider the preferred version of the page when determining rankings, rather than being confused by multiple versions of the same content.

Also, from the previous post on

Avoid Canonicalization

Definition: Canonicalization is the process of choosing the best URL from various url's (...if there are several choices for the url).

or

In general, it is the process of making something canonical-- that is, in conformance with some specification(according to rules). To canonicalize is to ensure that data conforms to canonical rules, and is in an approved format. Canonicalization may sometimes mean generating canonical data from noncanonical data(not according to rules).

Canonicalization is the process of standardizing and normalizing data to a consistent and uniform format. In the context of web development, canonicalization usually refers to the process of ensuring that there is only one URL that represents a particular page or resource on a website.

When a website has multiple URLs that lead to the same content, it can cause issues with search engines and affect the website's search engine rankings. For example, if a website has both a "www" and a "non-www" version of the same page, search engines may see this as duplicate content and penalize the website's rankings.

Canonicalization is important for ensuring that search engines recognize the preferred URL for a page or resource, and that all other URLs are treated as duplicates or variations. This can be achieved through various methods, such as setting a canonical tag in the HTML code of the page, using 301 redirects to redirect duplicate URLs to the preferred URL, or using server configurations to ensure that all requests are routed to the preferred URL.

For Example:

• www.sample.com
• sample.com/
• www.sample.com/index.html
• sample.com/home.asp

The above URL'S might appear same but to webserver they are completely different.


Things to be taken in consideration:

1. Pick the main URL & use it consistently in the website.
2. Use 301 redirect to main URL.
3. Don't remove www Vs non-www URL'S

Other ways that urls get canonicalized:

1. keeping or removing trailing slashes,
2. trying to convert urls with upper case to lower case,
3. removing session IDs from bulletin board or other software.