About Canonical Tags
What a canonical tag is for
A canonical tag names the version of a page you want indexed when the same content is reachable at more than one address. It is a statement of preference, and search engines usually honour it.
Duplicates arrive without anyone creating them: a trailing slash, http and https, www and bare, a tracking parameter, a printer-friendly version, a product reachable through two categories.
Left unresolved, those versions compete with each other and split the signals that should be accumulating on one page.
Getting it right
- Use an absolute URL, including the protocol.
- Point at a page that returns 200, not a redirect or a 404.
- Give every page a self-referencing canonical, including the preferred one.
- Keep it consistent with your sitemap and your internal links.
- One canonical per page. Two is treated as none.
Common mistakes
- Every page canonicalised to the homepage, which removes the whole site from results.
- Canonical pointing one way and a redirect pointing the other.
- Relative URLs, which resolve unpredictably.
- Canonicalising paginated pages to page one, hiding everything after it.
- Treating it as a directive. It is a hint, and a page whose content differs will be indexed anyway.
Canonicals and duplicate content
There is no penalty for duplicate content in the sense people usually mean. What happens is quieter: signals divide, the wrong version ranks, and neither performs as well as one consolidated page would.
That is the problem a canonical solves, and it is also why it is worth being deliberate rather than adding one everywhere by template and hoping.
Using the canonical tag generator
Enter the URL you want indexed and paste the tag into the head of every page that should defer to it - including that page itself.
Match it exactly to how the page is actually served, since a trailing slash or a missing www makes it a different URL.
Where to go next