In 2009, Yahoo, Microsoft, and Google introduced a new link type named canonical, which would quickly become one of the most important tools for SEOs, and since 2012 has been labelled as the official web standard in RFC 9596. As a component of a link element, it is implemented in the HTML header of a webpage, which is the same location that the title and the meta description can be found. If the canonical link is embedded on a page, it refers to a specified default URL or URI, which is also referred to as a canonical URL and is used as an indexing source instead of the page.
The canonical URL unites its own link popularity and reputation as well as that of the referenced pages – it generates a better ranking this way. However, since the tagged URLs are not included in the index, there are no problems with duplicate content. The label is merely a recommendation to the search engines, indicating that the link attribute does not necessarily have to be included in the index. If the implementation is incomplete or flawed, there is even the risk that the entire site will be ignored, which is why it’s so important to use the canonical tag correctly.