The canonical tag is an HTML element that enables webmasters to prevent duplicate content issues by specifying the "canonical", or "preferred", version of a web page.

Actually, the canonical tag is an attribute of the link tag:

In this example, the canonical tag tells search engines that 'http://www.example.com/page.htm' is the preferred URL for the page, even if the same page is available under several different URLs. The canonical tag can be added in the head section of a page.

The canonical attribute should only be used as a last resort. You should try to avoid duplicate content in the first place. Before adding the attribute, try the following to avoid duplicate content problems:

• Use 301 redirects to redirect old pages to their new versions.
• Use the robots.txt file to hide unwanted directories from search engines.
• Use CSS to create print or mobile versions of your web pages.

If your website uses faceted navigation, session ID's, etc. then you should use the canonical tag to show search engines the preferred URL of the page.

If you do not use the canonical tag correctly, your web page rankings might drop. Here are common errors:

• The canonical tag leads to a non-existing page.
• You use the same canonical tag with the same URL on all pages of your website.
• You put the canonical attribute in the body part of a web page instead of the head part.
• The canonical tag links to another website. This is correct when you syndicate your content on other websites. However, you should not use canonical tags with external links if you want to get high rankings for your own web pages.

When you copy pages on your website, make sure that you change the canonical tag. When you develop a new website, don't forget to remove placeholder URLs from the canonical tag.

Use absolute URLs in the canonical tag (https://www.example.com/page.htm) instead of relative URLs (/page.htm). Also, do not use the noindex tag on pages with the canonical tag. That can lead to indexation problems.

The canonical tag is not a directive. That means that search engines can ignore it. In the past, Google said that they see the canonical tag as a strong recommendation. If the linked pages aren't a close enough match, Google might ignore the tag.