The Quick Answer: A canonical tag is a line of code within the backend of a page that tells search engines which page you want to be indexed when multiple versions exist across different URLs. The Canonical Tag Was Announced February 2009. Google announced the launch of the canonical tag on its official blog on February 12th, 2009. They further explain the value of a canonical tag is to tell search engines which version of a URL is the one it should index and include in its search rankings. While Google highly encourages the use of the canonical tag, it