What Is a Canonical Tag? Definition & Explanation

A canonical tag is a piece of HTML code and points, for web pages with identical or almost identical content, to the main version. This way, search engine crawlers ignore duplicates. This is especially important for search engine optimization (SEO). However, there are a few aspects to consider when using canonical tags.

When a Canonical Tag Is Important

Crawlers of search engines like Google fundamentally orient themselves by URLs and not by web pages. That is why, for example, a filter function in an online shop can quickly lead to “duplicate content.”

This is because it creates different URLs such as

unternehmensname.de/Männermode/Sweatshirts.html
unternehmensname.de/männermode/Sweatshirts.html?Größe=M
unternehmensname.de/männermode/Sweatshirts.html?Größe=M&Farbe=Schwarz

Google sees each individual URL as its own page, even if the content barely differs.

Other typical use cases for a canonical tag are

Content in different formats, for example the original and a print version
Pages that appear under different categories, for example under “Tops” and “Sweaters”
Identical content on pages with and without www.
The same content with URL spellings with and without capital letters or slashes
The same content on HTTP and HTTPS
Search parameters
Standalone mobile web pages

Even when content appears on different websites, for example in the case of a guest post on a blog, it makes sense to use a canonical tag.

How to Set a Canonical Tag

There are two ways to set a canonical tag:

Option 1: Webmasters or site operators insert a link into the <head> of a page that should point to another. This link looks as follows:

<link rel=“canonical“ href=“https://www.beispielseite.de/original-url“ />

In the example above, on the page www.unternehmensname.de/männermode/Sweatshirts.html?Größe=M the tag would read:

<link rel=“canonical“ href=“https://www.unternehmensname.de/Männermode/Sweatshirts.html“ />

Option 2: The webmaster or site operator integrates the link into the HTTP response header.

Why Canonical Tags Are Important for SEO

If no canonical tag is set for duplicate content, there are the following dangers:

Google decides for itself which version it indexes. Then it may happen that a different page appears in the search results than website operators would like.
The search engine indexes multiple variants. This then often leads to keyword cannibalization.
It is unclear to Google whether link equity should be distributed or consolidated on one page.

One possible consequence is a worse ranking. Apart from that, users may reach the “print” version of a page via Google search.

A canonical tag brings an additional benefit with it: crawlers need less budget for web pages. However, this only matters for large websites. There is an important limitation to note: for Google, the canonical tag has the status of a hint (signal). The search engine can follow it, but is not obligated to. To choose the canonical URL, Google draws on over 40 signals – including internal linking, sitemap entries, redirects, and HTTPS. If these signals deviate strongly from the set canonical, Google can override the tag and classify a different URL as canonical.

Using Canonical Tag, noindex, or 301 Redirect Correctly

A canonical tag bundles ranking signals onto a main version while all variants remain accessible. This is especially suitable for filter and parameter pages as well as for identical content under multiple URLs. If a page should not appear in the index at all, noindex is the appropriate means. If a page permanently no longer exists or has moved, a 301 redirect belongs there. Canonical, noindex, and 301 redirect solve different tasks and should therefore not be combined with one another.

It is helpful here to separate the levels: noindex and nofollow are robots directives that are set via the meta robots tag or the X-Robots-Tag in the HTTP header. Disallow, on the other hand, belongs in the robots.txt and controls crawling; it does not directly affect indexing.

Tips for Setting Canonical Tags Correctly

To achieve the desired result, it is important to consider a few things when using canonical tags:

The pages should be identical at least in large parts. Otherwise, there is a risk that the crawler will ignore the tag or even all canonical tags on the page.
The same happens if there are multiple canonical tags on one page.
A canonical tag should not be combined with noindex, disallow, or nofollow.
Errors frequently occur in the spelling of the web pages a canonical tag links to. Among other things, you should pay attention to absolute URLs (with http:// or https://).

Self-Referencing Canonical Tags as the Standard

Sometimes site operators are not even aware that multiple versions of a page exist. With self-referencing canonical tags, they also cover unnoticed duplicates by having the tag point to the page itself.

This self-referencing canonical is now considered a standard best practice. Every indexable page should have a canonical that points to itself. This way, site operators actively determine which URL is the canonical one and prevent Google from independently selecting a different version in the case of unnoticed URL variants – such as parameters, capitalization, or trailing slashes.

Checking the Canonical URL Chosen by Google

In the Google Search Console, the URL inspection shows which version Google actually uses: under the entries “User-declared canonical” and “Google-selected canonical.” If the two differ, this indicates contradictory signals. It is also useful to compare raw HTML and rendered HTML, since canonicals set client-side via JavaScript do not always take effect reliably.

Cross-Domain Canonical and Multilingual Pages

For a guest post that has the same content on an external domain, a cross-domain canonical can point to the original. In multilingual setups, the canonical for each language version points to itself (self-referencing), while the language assignment runs via hreflang markup. Canonical and hreflang should not contradict each other, as Google will otherwise ignore the signals.

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