A canonical URL allows you to tell search engines that certain similar URLs are actually one and the same.
Sometimes you have products or content that is accessible under multiple URLs, or even on multiple websites.
Using a canonical URL (an HTML link tag with attribute rel=canonical) these can exist without harming your rankings. What is the canonical link element?
The SEO benefit of rel=canonical
The process of canonicalization
How to set canonical URLs
Correct example of using rel=canonical
Setting the canonical in Yoast SEO
When should you use canonical URLs?
301 redirect or canonical?
Should a page have a self-referencing canonical URL?
Cross-domain canonical URLs
Faulty canonical URLs: common issues
rel=canonical and social networks
Advanced uses of rel=canonical
Canonical link HTTP header
Using rel=canonical on not so similar pages
Using rel=canonical in combination with hreflang
Conclusion: rel=canonical is a power tool
What is the canonical link element?
History of rel=canonical
In February 2009 Google, Bing and Yahoo! introduced the canonical link element.
Matt Cutt’s post is probably the easiest reading if you want to learn about its history.
While the idea is simple, the specifics of how to use it turn out to be complex.
The rel=canonical element, often called the “canonical link”, is an HTML element that helps webmasters prevent duplicate content issues.
It does this by specifying the “canonical URL”, the “preferred” version of a web page.
Using it well improves a site’s SEO.
The idea is simple: if you have several similar versions of the same content, you pick one “canonical” version and point the search engines at that.
This solves the duplicate content problem where search engines don’t know which version of the content to show.
This article takes you through the use cases and the anti-use cases.