What Is a Canonical URL?
You usually reach a website’s homepage at one obvious address. But behind the scenes, the very same content is often reachable at several slightly different URLs: with or without www, with a trailing slash, with a tracking parameter, or as a print view. Search engines dislike that, because they want to show one best URL per piece of content — and they have to choose when they find duplicates. The canonical URL is how you tell them which one is the original.
Duplicates come from small differences
Classic duplicate-triggering variations:
https://site.com/pagevshttps://site.com/page/(trailing slash)https://www.site.com/pagevshttps://site.com/page(www)https://site.com/page?utm_source=x(tracking parameters)https://site.com/page#print(fragment or print view)- Syndicated or republished copies of the same article
Each of these is a separate URL to a crawler, even though a human sees the same page. When search engines find several versions, none of them passes the full value of the content on to the others — they split the attention instead.
How the canonical tag works
In the <head> of each page you can declare the original version with a single line:
<link rel="canonical" href="https://site.com/page" />On the original page itself, the tag should point to itself (a self-referencing canonical). On the duplicates, it points to the original. Crawlers treat it as a strong instruction: index this URL, and treat the duplicates as versions of it.
Canonical vs redirect: pick the right tool
The two mechanisms solve related but different problems:
- 301 redirect: the duplicate URL stops existing. Visiting it sends you to the target. Use this when you want only one URL to exist — for example after a domain change or a permanent restructure.
- Canonical: both URLs keep working and can both be visited, but one is declared the representative. Use this when both must remain accessible — a print view, a mobile version that still needs a separate URL, or parametered pages that do real work.
A common mistake is using a canonical where a redirect would be simpler, or vice versa. If users legitimately land on both URLs and both should work, canonical is right. If one URL should simply not exist anymore, redirect.
When canonicals break
Canonical tags fail quietly. Common failure modes:
- The tag points to an error or redirecting URL.
- There is no self-referencing canonical, so every URL declares a different original.
- A plugin or template injects a canonical pointing at a different domain.
- The tag is missing entirely on pages that exist in multiple forms.
The SEO Checker reads the actual canonical from the live HTML of any page and reports whether it matches the URL you checked — the fastest way to spot these failures.
Verify with the tools
- Run the SEO Checker on your homepage and on a parametered URL if you have one.
- Check the same page with and without
wwwusing the HTTP Status Checker and confirm the redirect chain ends at one canonical host. - Open your site’s sitemap and confirm each listed URL returns
200, not a redirect to a different host.