What is canonicalization

Canonicalization is the process of selecting the representative
–canonical– URL of a piece of content. Consequently, a canonical
URL is the URL of a page that Google chose as the most representative from a set of duplicate
pages. Often called deduplication, this process helps Google show only one version of the
otherwise duplicate content in its search results.

There are many reasons why a site may have duplicate content:

• Region variants: for example, separate USA and UK URLs with
  hreflang, but
  essentially the same content in the same language
• Device variants: for example, a page with both a mobile and a desktop version
• Protocol variants: for example, the HTTP and HTTPS versions of a site
• Site functions: for example, the results of sorting and filtering functions of a category
  page
• Accidental variants: for example, the demo version of the site is accidentally left
  accessible to crawlers

Some duplicate content on a site is normal and it’s not a violation of
Google’s spam policies. However, having the
same content accessible through many different URLs can be a bad user experience (for example,
people might wonder which is the right page, and whether there’s a difference between the two) and
it may make it harder for you to track how your content performs in search results.

How Google indexes and chooses the canonical URL

When Google indexes a page, it
determines the primary content (or centerpiece) of each page. If Google finds
multiple pages that seem to be the same or the primary content very similar, it clusters them
together. Google then chooses the page that, based on the factors (or signals) the
indexing process collected, is objectively the most complete and useful for search users, and
marks it as the canonical. The canonical page will be crawled most regularly; duplicates are
crawled less frequently in order to reduce the crawling load on sites.

There are a handful of factors that play a role in canonicalization: whether the page
is served over HTTP or HTTPS, redirects, presence of the URL in a sitemap, and
rel="canonical" link annotations. 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. That is, indicating a canonical preference is a hint, not a rule.

Different language versions of a single page are considered duplicates only if the primary
content is in the same language (that is, if only the header, footer, and other non-critical
text is translated, but the body remains the same, then the pages are considered to be
duplicates). For regional variants in the same language (for example, a piece of content for the
USA and the UK that’s accessible at different URLs but in the same language),
use both canonicalization and hreflang
to help Google better understand which regional URL to show in search results. To learn more, see our
documentation on
handling duplicate pages on multi-regional sites.

Google uses the canonical page as the main source to evaluate content and quality. A Google
Search result usually points to the canonical page, unless one of the duplicates is explicitly
better suited for a search user. For example, the search result will probably point to the
mobile page if the user is on a mobile device, even if the desktop page is the canonical.

Read more about
how to indicate your preference for the canonical URL, and whether you need to.