A canonical URL is a marker that tells Google which version of a page is the "primary" one, and it was jointly introduced by Google, Yahoo and Microsoft back in 2009. On the modern web almost every piece of content is reachable through several different URLs — http and https, www and non-www, with tracking parameters and without them. When search engines treat these variations as separate pages, the link equity gets divided between them and the overall ranking power drops noticeably.
The duplicate content problem
Google does not officially apply a penalty for duplicate content, yet in practice pages with identical content end up competing against each other in the search results. The search engine independently decides which version to display, and this is often not the URL you would prefer to promote. As a result external backlinks scatter across multiple addresses and no single version accumulates enough authority to reach the top positions.
How to write the rel=canonical tag
The canonical tag belongs inside the head section of your HTML document and points to the preferred URL of the page. The most important rule is that every page should have a self-referencing canonical pointing back to itself. This clearly communicates the correct address to Google and consolidates all parameterised variants with the main version of the page.
Self-referencing and cross-domain canonicals
The self-referencing canonical is the most common scenario and is recommended for every page on your site. A cross-domain canonical points to a page on a different domain and is typically used during content syndication. If your article gets republished on another publication, that publication can use canonical to credit the original source and pass the ranking signals back to it.
E-commerce and parameterised URLs
Category pages on online stores tend to explode into hundreds of variations due to filters, sorting options and pagination. Parameters such as ?color=red, ?sort=price or ?page=2 create distinct URLs for what is essentially the same product listing. In these situations the canonical of every filtered variant should point to the clean category URL without parameters. UTM tags and session identifiers are handled in exactly the same way to keep marketing links from generating duplicates.
www, https and trailing slashes
The same website can be served on four different versions: http://sayt.uz, http://www.sayt.uz, https://sayt.uz and https://www.sayt.uz. Google may potentially treat each of these as a separate page with its own ranking signals. The correct approach is to pick one canonical version, set up 301 redirects from the others and use canonical to explicitly confirm the chosen variant. The trailing slash question is solved by the same logic.
Common mistakes and verification
When configuring canonical tags the most frequent mistakes include pointing to a 404 page, creating a chain of redirects, using the same URL on every page of the site and combining canonical with noindex on the same page. The Google Search Console "Pages" report shows the canonical that Google itself selected for each URL, and it must match the one you specified. A 301 redirect is appropriate when content has permanently moved, while canonical is used when both URLs need to remain accessible to users.