Google may select a canonical URL based on your site’s perceived preference and the URL that is more useful for the user, Webmaster Trends Analyst John Mueller explained on the September 9 edition of #AskGoogleWebmasters.

The question. “You can indicate your preference to Google using these techniques, but Google may choose a different page as canonical than you do, for various reasons. So, what are the reasons? Thanks!” user @uale75 asked via Twitter.

The answer. Before addressing the question, Mueller contextualized it by providing examples of common configurations in which multiple unique URLs lead to the same content, such as when the homepage is accessible as index.html or when both lower and uppercase versions of a URL direct to the same page.

“For search, it doesn’t make much sense to index and show all of these versions, so we try to pick one and focus on that,” Mueller explained. “We try to pick the canonical URL by following two general guidelines: First, which URL does it look like the site want us to use; so, what’s the site’s preference? And secondly, which URL would be more useful for the user?”

In terms of the “site’s preference,” Mueller said that Google takes into account a number of factors, including:

• Link rel canonical annotation.
• Redirects.
• Internal linking.
• The URL in the sitemap file.
• HTTPS URLs over HTTP URLs.
• The “nicer-looking” URL.

“For all these, we consider the factors involved for each potential canonical URL and then pick the one where more things come together.”

“If you’re a site owner and you have a strong preference regarding URLs that you want to have shown to users in search, first of all, you should make sure that you use those preferences consistently across your website,” he advised. “Ideally, search engines wouldn’t even be able to stumble across any of those alternatives — if you have a preference, then stick to it.”

What happens when a different URL is chosen? “Simply put, it’s just the URL that’s shown in search. If our systems pick a different URL as a canonical, it’ll rank just the same in search. In the end, it really just comes down to your preference,” Mueller stated, adding, “If a different URL happens to be chosen from time to time, that’s not going to negatively affect the site either.”

Why we should care. Using the rel=canonical attribute and consolidating your signals by standardizing URL and linking practices across your site helps you keep duplicate content under control and may also help to ensure that search engines point users to the pages you intended. 

Learn more about the canonical tag. Here are a few additional resources that can help you determine when to use the canonical tag.

• Canonical tags gone wild
• SMX replay: SEO that Google tries to correct for you
• Canonical chaos: doubling down on duplicate content
• Canonical tags are easy, right? What’s the worst that could happen?