What is a Canonical tag?
The rel="canonical" link element can be used to specify the canonical URL of a webpage.
Known as various names: canonical tag, rel canonical tag etc.
Syntax
Below is example syntax for a rel=canonical link, where URL is the URI of the web-page the canonical tag is referencing.
<link rel="canonical" href="URL" />
Examples
Below is an example of rel="canonical" tag for the secure/HTTPS non-www homepage of the example.com website.
<link rel="canonical" href="https://example.com/" />
Example of a rel=canonical tag pointing at a specific page:
<link rel="canonical" href="https://example.com/example-page/" />
Why are canonical tags important?
Using a canonical tag is it possible to prevent canonicalisation and duplicate content related issues.
Simply put, canonical tags allow you specify the URL/domain/version of a webpage you want search engines to index.
Web resources can often be available at different domains/sub-domains and protocols, eg:
- http://www.example.com
- http://example.com
- https://www.example.com
- https://example.com
Sometimes a resource can also have multiple paths, such as:
- https://www.example.com/product/red-dress/
- https://www.example.com/product/red-dress/index.html
- https://www.example.com/product/category/red-dress/
- https://www.example.com/red-dress/
Correctly using a canonical tag, such as in the above examples, in theory prevents search engines from indexing a resource using the wrong format.