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A canonical tag (aka "rel canonical") is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or "duplicate" content appearing on multiple URLs. Practically speaking, the canonical tag tells search engines which version of a URL you want to appear in search results.

Briefly the rel=canonical tag is a way to tell Google that one URL is equivalent to another URL, for search purposes. Typically, a URL (B) is a duplicate of URL (A), and the canonical tag points to (A). The following tag would appear on the page that generates URL (B), in the :

<link rel=”canonical” href=”http://www.example.com/url-a.html” />

I'm wondering what will be the SEO impact if the canonical link is returning 404 and not exists.

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  • Is this theoretical, or is that reason that a site would have canonical links to a 404 not found page? – Stephen Ostermiller♦ Sep 18 '17 at 16:40
  • no it's not theoretical. I'm wondering what is happening if I put a wrong canonical link to my page? – Hazem Hagrass Sep 18 '17 at 17:14
  • I'm not sure exactly how Google handles that specific situation. I know that Google is sometimes now ignoring canonical tags when it thinks they are wrong. It used to be that putting bad canonicals in would completely kill your SEO. Now days Google seems to be doing more sanity checks before honoring canonical tags. – Stephen Ostermiller♦ Sep 18 '17 at 17:16
  • Are you sure that nowadays google are be down my some sanity checks? @StephenOstermiller any reference for that – Hazem Hagrass Sep 18 '17 at 18:05
  • We've had some questions here from users that have wondered why canonicals are not seeming to be honored. Google has also invited me to the beta of the new Search Console reporting. The new "index coverage" report shows URLs that are excluded from the Google index. One reason Google lists for excluding URLs is that "Google chose different canonical than user." In my case that appears to be because I recently canonicalized to HTTPS, but Google is still indexing some pages with HTTP. – Stephen Ostermiller♦ Sep 18 '17 at 18:12
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Consider www.example.com/abc and www.example.com/us/xyz. Both pages are same with exact duplicate content. In case you want /us/xyz to rank in SERPs.

Google Webmaster Tools recommends 301 redirection of /abc to /us/xyz to avoid duplication penalty or ranking drop.

In some cases 301 redirection is not possible due to technical limitations. Then Google advises to place rel canonical tag - <link rel=”canonical” href=”http://www.example.com/us/xyz”/> in the source code of www.example.com/abc. This way you intimate the bots that www.example.com/us/xyz is the actual page that needs to rank. www.example.com/abc will still be crawled but the link juice and authority moves to www.example.com/us/xyz and it will start ranking for relevant results based on your site Authority.

In case you place an incorrect URl in canonical tag, both pages will lose on visibility on SERPs. In some case it will still show /abc if the page authority is very high against the query searched by user. But your competitor is surely going to gain from it.

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  • Do you have recent references or personal experience that leads you to conclude that pages with incorrect canonicals cause problems in the SERPs? While I would have agreed with this answer a few years ago, I think Google is now doing a better job of ignoring incorrect canonicals. – Stephen Ostermiller♦ Sep 20 '17 at 17:52
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Essentially, the canonical url tag would be telling search engines that the definitive url that you want to rank is the one that triggers a 404.

When crawlers follow this canonical url tag, and see the 404 status, they're going to go away and maybe come back again later (as the 404 is just 'temporary' rather than a 410 'gone'), but the lack of any quality content on the 404 page will eventually see the canonical url depreciated in search results.

I'd recommend that you really sort out the canonical > 404 issue, either by choosing a different canonical url (one that exists), or by creating the missing page.

If your actual goal is to get rid of a page, then use the 410 'Gone' status rather than the 404, and don't bother to point a canonical at it.

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