Should every page have a canonical tag?

Yes, it's good practice for every page to have a self-referencing canonical. It protects against duplicates from parameters, trailing slashes, and http vs https.

Which is better: a canonical or a 301 redirect?

A redirect when users no longer need the old URL. A canonical when both URLs should keep working, such as a page with and without filters.

Can a canonical point to another domain?

Yes — for example when the same article is syndicated on several sites. The page pointing to another domain usually drops out of the index in favour of the target.

Canonical tag (rel=canonical)

A canonical tag is <link rel="canonical" href="…"> in the page head that tells search engines which URL is the main version of a page. When the same content is available at several URLs, Google usually indexes the one named in the canonical. It's a hint, not a directive — Google can choose differently.

What a canonical looks like

In the page <head>:

<link rel="canonical" href="https://shop.com/sofas/">

You can send the same signal in an HTTP header (Link: <https://shop.com/file.pdf>; rel="canonical") — useful for PDFs. The canonical URL should be absolute, with protocol and domain.

When to use it

parameter URLs (filters, sorting, campaign tracking) point to the clean URL,
the same content in several categories or under several paths,
print versions, AMP, product variants with identical descriptions,
every page: a self-referencing canonical as a safeguard.

Common mistakes

Mistake	Result
homepage canonical on every page	subpages drop out of the index
canonical to a URL that redirects or returns 404	Google ignores the hint or chooses itself
canonical to the http version instead of https	signals go to the wrong version
two different canonicals on one page	Google ignores both
canonical and noindex on the same page	conflicting signals
canonical added only by JavaScript	the crawler may miss it

How to check the canonical

Open the page source and search for rel="canonical". In Google Search Console, URL Inspection shows both the user-declared and the Google-selected canonical. The redirect checker shows the canonical of the page where the chain ends.

Monitoring canonicals

Canonical changes are rarely intentional — they usually arrive with a template or SEO plugin update.

IndexRise records the canonical of every monitored page. When it starts pointing to another URL, you get a critical "Page is no longer indexable" event. When it changes but still points to the same page (for example a relative instead of a full URL), you only get a warning.