What it is
Most websites can technically be reached through more addresses than anyone intended. Depending on how your domain and server are configured, all of the following might load the exact same homepage: https://yourdomain.com, https://www.yourdomain.com, http://yourdomain.com, and https://yourdomain.com/ with a trailing slash added. Each of those is a technically distinct web address, even though a human visitor would never notice a difference between them.
"Canonicalization" is the general term for picking one of those addresses as the official, preferred one and making sure everything — your internal links, your server configuration, and a canonical tag in your page's code — consistently points there instead of leaving all the variants floating around as equally valid.
Why it matters
Left unresolved, these URL variants split what should be one single page into what looks, from a technical standpoint, like several separate pages that all happen to share identical content. That's a genuinely confusing situation for software to interpret cleanly, and it's the kind of quiet, structural mess that's far easier to prevent from the start than to untangle later once links to multiple versions have spread across the internet.
This is a deeper, more technical version of the canonical tag question — it's less about the one line of code and more about the underlying server and domain configuration that determines which address variants actually work at all.
How to fix it
- Decide on one final, preferred version of your domain: with or without
www, and check whether your important pages should end in a trailing slash or not. - In your domain or hosting settings, set up a server-level redirect so every other variant (the non-preferred www version, and the plain http:// version) automatically forwards to your one preferred address.
- Make sure every internal link on your own site (navigation menus, buttons, footer links) points at the preferred version directly, rather than relying on a redirect to fix it after the fact.
- Add a self-referencing canonical tag to each page (see our canonical tag guide) as a second layer of protection on top of the redirects.
- Test by typing a couple of the non-preferred variants into a browser and confirming each one redirects automatically to your chosen address.