What does a canonical tag do?

It tells search engines which URL you consider the main version of a page when several URLs show the same or similar content. Google treats it as a strong hint, not a command.

Is a missing canonical tag a problem?

Not always. Google will pick a canonical itself. Adding a self-referencing canonical removes the guesswork, especially on pages reachable with tracking parameters or several URL variants.

Why would a canonical point to another page?

On purpose for duplicates, such as print versions or filtered product lists. By mistake when a template copies the homepage canonical onto every page, which tells Google to ignore those pages.

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Free analysis

Free tools

Canonical tag checker

Enter up to ten URLs to see each page's canonical tag and whether it points to itself, is missing, or points to a different URL.

The mistake worth checking for

A template or plugin that sets every page's canonical to the homepage. Each page then tells Google it is a copy of the homepage, and those pages can drop out of results. This checker shows it at a glance: every row reads “points to a different URL”.

Rules that keep canonicals working

Use absolute URLs, on the final https host your site redirects to.
Point to a page that returns 200 and is not set to noindex.
Keep the canonical, internal links and sitemap consistent with each other.

Google's guide to consolidating duplicate URLs covers the details. Check redirects with the redirect checker and the rest of the head with the meta tag checker.

Questions

What does a canonical tag do?: It tells search engines which URL you consider the main version of a page when several URLs show the same or similar content. Google treats it as a strong hint, not a command.
Is a missing canonical tag a problem?: Not always. Google will pick a canonical itself. Adding a self-referencing canonical removes the guesswork, especially on pages reachable with tracking parameters or several URL variants.
Why would a canonical point to another page?: On purpose for duplicates, such as print versions or filtered product lists. By mistake when a template copies the homepage canonical onto every page, which tells Google to ignore those pages.

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