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Canonicalization

How to specify a canonical URL with rel=”canonical” and other methods

To specify a canonical URL for
duplicate or very similar pages to Google Search, you can indicate your preference using a
number of methods. These are, in order of how strongly they can influence canonicalization:

Keep in mind that these methods can stack and thus become more effective when combined.
This means that when you use two or more of the methods, that will increase the chance
of your preferred canonical URL appearing in search results.

While we encourage you to use these methods, none of them are required; your
site will likely do just fine without specifying a canonical preference. That’s because
if you don’t specify a canonical URL,
Google will identify which version of the URL is objectively the best version to show to users in Search.

Reasons to specify a canonical URL

While it’s generally not critical to specify a canonical preference for your URLs, there
are a number of reasons why you would want to explicitly tell Google about a canonical
page in a set of duplicate or similar pages:

  • To specify which URL that you want people to see in search results.
    You might prefer people to reach your green dress product page through
    https://www.example.com/dresses/green/green-dress.html rather than
    https://example.com/dresses/cocktail?gclid=ABCD.
  • To consolidate signals for similar or duplicate pages. It helps
    search engines to be able to consolidate the signals they have for the individual
    URLs (such as links to them) into a single, preferred URL. This means that signals
    from other sites to https://example.com/dresses/cocktail?gclid=ABCD get
    consolidated with links to
    https://www.example.com/dresses/green/green-dress.html if the latter
    becomes canonical.
  • To simplify tracking metrics for a piece of content. With a variety
    of URLs, it can be more challenging for you to get consolidated metrics for a
    specific piece of content.
  • To avoid spending crawling time on duplicate pages. You may want
    Googlebot to get the most out of your site, so it’s better for it to spend time
    crawling new (or updated) pages on your site, rather than crawling duplicate
    versions of the same content.

Best practices

For all canonicalization methods, follow these best practices:

  • Don’t use the robots.txt file for canonicalization purposes. Google may still index URLs
    that are disallowed in robots.txt without their content.
  • Don’t use the URL removal tool for canonicalization. It hides
    all versions of a URL from Search.
  • Don’t specify different URLs as canonical for the same page using
    different canonicalization techniques (for example, don’t specify one URL in a
    sitemap, but a different URL for that same page using rel="canonical").
  • Don’t specify a URL fragment as canonical, as
    Google generally doesn’t support URL fragments.
  • Do include a rel="canonical" link on the canonical page
    itself (also known as a self-referential canonical).
  • We don’t recommend using
    noindex to prevent
    selection of a canonical page within a single site, because it will completely block the page
    from Search. rel="canonical" link annotations are the preferred
    solution.
  • If you’re using
    hreflang elements,
    make sure to specify a canonical page in the same language, or the best possible
    substitute language if a canonical page doesn’t exist for the same language.
  • When linking within your site, link to the canonical URL rather than a duplicate URL.
    Linking consistently to the URL that you consider to be canonical helps Google
    understand your preference.
  • If you’re using client-side rendering with JavaScript, it’s important to make sure that the
    information about the canonical URL is as clear as possible. The best way to do this is to
    specify the canonical URL in the HTML source code and make sure that JavaScript doesn’t change
    the canonical link element. If you can’t set the canonical URL in the HTML source code, leave it
    out and only set it with JavaScript. This ensures that the information about the canonical URL
    is as clear as possible.

Comparison of canonicalization methods

The following table compares the different canonicalization methods, highlighting their
strengths and weaknesses when it comes to maintenance and efficacy in different scenarios.

Method and description
rel="canonical" link element

Add a <link> element in the code for all duplicate pages,
pointing to the canonical page.

Pros:
  • Can map an infinite number of duplicate pages.
Cons:
  • Can be complex to maintain the mapping on larger sites, or sites where
    the URLs change often.
  • Only works for HTML pages, not for files such as PDF. In such cases,
    you can use the rel="canonical" HTTP header.
rel="canonical" HTTP header

Send a rel="canonical" header in your page response.

Pros:
  • Doesn’t increase page size.
  • Can map an infinite number of duplicate pages.
Cons:
  • Can be complex to maintain the mapping on larger sites, or sites where
    the URLs change often.
Sitemap

Specify your canonical pages in a sitemap.

Pros:

  • Simple to implement and maintain, especially on large sites.

Cons:

  • Google must still determine the associated duplicate for any
    canonicals that you declare in the sitemap.
  • Less powerful signal to Google than the rel="canonical"
    mapping technique.
RedirectsUse permanent redirects to tell Google that a redirected URL is a worse version than the URL
it redirects to. Use this only when deprecating a duplicate page.
AMP variantIf one of your variants is an AMP page, follow the AMP guidelines to
indicate the canonical page and AMP variant.

Google supports explicit rel canonical link
annotations as described in
RFC 6596.
rel="canonical" annotations that suggest alternate versions of a page are ignored;
specifically, rel="canonical" annotations with hreflang,
lang, media, and type attributes are not used for
canonicalization. Instead, use the appropriate link annotations to specify alternate
versions of a page; for example, link rel="alternate"
hreflang for
language and country annotations.

You can provide the rel="canonical" link annotations in two ways:

We recommend that you choose one of these and go with that; while supported, using
both methods at the same time is more error prone (for example, you might provide
one URL in the HTTP header, and another URL in the rel="canonical"
link element).

A rel="canonical" link element (also known as a
canonical element) is an element used in the head section of HTML
to indicate that another page is representative of the content on the page.

Suppose you want https://example.com/dresses/green-dresses to be the
canonical URL, even though a variety of URLs can access this content. Indicate this
URL as canonical with these steps:

  1. Add a <link> element with the attribute
    rel="canonical" to the <head> section of
    duplicate pages, pointing to the canonical page. For example:
  2. If the canonical page has a mobile variant on a separate URL, add a
    rel="alternate" link element to it, pointing to the mobile
    version of the page:
  3. Add any
    hreflang
    or other elements that are appropriate for the page.

Use absolute paths rather than relative paths with the rel="canonical"
link element. Even though relative paths are supported by Google, they
can cause problems in the long run (for example, if you unintentionally allow your
testing site to be crawled) and thus we don’t recommend them.

Good example:
https://www.example.com/dresses/green/green-dress.html

Bad example:
/dresses/green/green-dress.html

The rel="canonical" link element is only accepted if it
appears in the <head> section of the HTML, so make sure at least the
<head> section is valid HTML.

If you use JavaScript to add the rel="canonical" link element,
make sure to
inject the canonical link element properly.

The rel="canonical" HTTP header

If you can change the configuration of your server, you can use a link
HTTP response header
with a rel="canonical" target attribute as defined by
RFC5988
rather than an HTML element to indicate the canonical URL for a document supported
by Search, including non-HTML documents such as PDF files.

Google supports this method for web search results only.

If you publish content in many file formats, such as PDF or Microsoft Word, each on
their own URL, you can return a rel="canonical" HTTP header to tell
Googlebot what is the canonical URL for the non-HTML files. For example, to indicate
that the PDF version of the .docx version should be canonical, you might
add this HTTP header for the .docx version of the content:

As with the rel="canonical" link element, use absolute URLs
in the rel="canonical" HTTP header.

Use a sitemap

Pick a canonical URL for each of your pages and submit them in a
sitemap. All pages
listed in a sitemap are suggested as canonicals; Google will decide which pages
(if any) are duplicates, based on similarity of content.

Supplying the preferred canonical URLs in the sitemaps is a straightforward way of defining
canonicals for a large site, and sitemaps are a useful way to tell Google which pages
you consider most important on your site.

Use redirects

Use this method when you want to get rid of existing duplicate pages. All
permanent redirection methods
have the same effect on Google Search, however the time it takes for search engines to notice the
different redirect methods may differ.

For the quickest effect, use HTTP (also known as server-side) redirects.

Suppose your page can be reached in multiple ways:

  • https://example.com/home
  • https://home.example.com
  • https://www.example.com

Pick one of those URLs as your canonical URL, and use redirects to send traffic from
the other URLs to your preferred URL.

Other signals

Apart from explicitly provided methods, Google also uses a set of canonicalization signals
that are generally based on site setup: preferring HTTPS over HTTP, and URLs in
hreflang clusters.

Prefer HTTPS over HTTP for canonical URLs

Google prefers HTTPS pages over equivalent HTTP pages as canonical, except when there
are issues or conflicting signals such as the following:

  • The HTTPS page has an invalid SSL certificate.
  • The HTTPS page contains insecure dependencies (other than images).
  • The HTTPS page redirects users to or through an HTTP page.
  • The HTTPS page has a rel="canonical" link
    to the HTTP page.

Although our systems prefer HTTPS pages over HTTP pages by default, you can ensure this
behavior by taking any of the following actions:

  • Add redirects from the HTTP page to the HTTPS page.
  • Add a rel="canonical" link from the HTTP
    page to the HTTPS page.
  • Implement
    HSTS.

To prevent Google from incorrectly making the HTTP page canonical, avoid
the following practices:

  • Avoid bad TLS/SSL certificates and HTTPS-to-HTTP redirects because they cause Google
    to prefer HTTP very strongly. Implementing HSTS cannot override this strong preference.
  • Don’t include the HTTP version of your pages in your sitemap or
    hreflang annotations
    rather than the HTTPS version.
  • Avoid implementing your SSL/TLS certificate for the wrong host-variant. For example,
    example.com serving the certificate for subdomain.example.com.
    The certificate must match your complete site URL, or be a wildcard certificate that
    can be used for multiple subdomains on a domain.

Prefer URLs in hreflang clusters

To help with sites’ localization efforts, for canonicalization purposes Google prefers
URLs that are part of hreflang clusters. For example, if
https://example.com/de-de/cats and
https://example.com/de-ch/cats reciprocally point to each other with
hreflang annotations, but not to
https://example.com/de-at/cats, the pages for
de-de and de-ch will be preferred as canonicals instead
of the /de-at/ page that doesn’t appear in the
hreflang cluster.

Read more about
troubleshooting and fixing canonicalization issues.

Alien Road

So setzen wir das um

We use rel=canonical, consistent internal links and a single sitemap entry as one coherent signal set. Contradicting yourself across those three is why Google picks a URL you did not intend.

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