A canonical tag, also known as a canonical link or canonical URL, is a HTML element, which is in the <head> area of a website. It indicates Search engines The „original“ or „main“ version of a page is the version that Google considers to be the "original" or "main" version of identical or very similar content. This tag is used with the attribute rel="canonical" and refers to the preferred URL to be indexed.
The importance of the canonical tag for SEO
The primary function of the canonical tag is to solve problems with Duplicate content (duplicate content). Duplicate content occurs when the same or very similar content is accessible via different URLs. This can occur internally on your own domain or externally on several domains.
Such duplicates can pose challenges for search engines:
- Waste of resources: Crawler have to search and index an unnecessarily large number of identical pages, which reduces the crawl budget.
- Ranking dilution: If several pages have the same content, they share the „Link power“ and ranking signals instead of bundling them on a single, authoritative page. This can lead to poorer visibility in the Search results lead.
- Incorrect indexingSearch engines may have difficulty deciding which version of a page to display in search results, or even prioritize the „wrong“ version.
By using the canonical tag correctly, a webmaster clearly signals which URL represents the authoritative version of a piece of content. This ensures that search engines index this preferred URL, consolidate the link signals and display the desired page in the search results. This optimizes crawling, protects against potential ranking losses and improves the overall search engine optimization (SEO) of a website.
Implementation and use cases
The canonical tag is used in the <head>-area of the HTML document of a „copy“ page and refers to the „original“ page. An example of the implementation looks like this:
<link rel="canonical" href="https://www.ihredomain.de/originalseite/" />It is crucial to specify only one canonical URL per page. If several canonical tags are found in the source text, they will be ignored by search engines. In addition, the canonical tag should always refer to an absolute URL and not to a URL that triggers a 404 status code.
Common scenarios in which the canonical tag is used are:
- Online storesVariants of products (e.g. different colors or sizes) that are accessible via separate URLs but have identical product descriptions.
- URL parametersPages with tracking codes, session IDs or filter options that lead to different URLs with the same content.
- Technical duplicatesIf a website is accessed via different protocols (HTTP vs. HTTPS), subdomains (www. vs. without www.) or file names (e.g.
/index.html) can be reached. - Pagination: For paginated pages, it is usual to provide each page with a self-referencing canonical tag to prevent problems.
- Cross-domain content: When publishing the same content on several domains, for example with Guest contributions or syndication.
Even if there is no obvious duplicate content, it is advisable to set a self-referencing canonical tag on each page. This signals Google clearly identifies the preferred version and proactively protects against unintentional duplicates due to technical factors.
