Prevent duplicate content with Canonical Tags
SEO is not just about good keywords and backlinks, but also about technical optimization. An important part of this is the use of the canonical tag. In this article we explain in a simple way what a canonical tag is, why it is important for your findability in Google, and how to apply it correctly. We also provide practical HTML examples that you can use right away.
What is a canonical tag?
A canonical tag is an HTML element that you add to a web page to make it clear to search engines what the original or "preferred" version of a page is. It helps prevent multiple pages with the same (or almost the same) content from being seen as duplicate content.
Why is a canonical tag important for SEO?
Duplicate content can harm your rankings because search engines do not know which version to show in the search results. By using a canonical tag, you indicate which page is the most important. This ensures that:
- Your link value is not fragmented across multiple URLs
- You avoid SEO penalties for duplicate content
- You maintain control over which pages are indexed
What does a canonical tag look like?
A canonical tag is added to the <head> of your HTML and looks like this:
<link rel="canonical" href="https://www.yoursite.com/original-page" />Example 1: Same content via multiple URLs
Suppose you sell a product that can be reached via multiple URLs:
- https://www.jouwsite.nl/product?kleur=rood
- https://www.jouwsite.nl/product?kleur=blauw
Then you indicate that the original version is, for example:
<link rel="canonical" href="https://www.jouwsite.nl/product" />Example 2: Print-friendly version of a page
Sometimes you have a separate URL for print versions, for example:
- https://www.jouwsite.nl/artikel
- https://www.jouwsite.nl/artikel/print
On the print version you then place:
<link rel="canonical" href="https://www.jouwsite.nl/artikel" />Example 3: Pages with sorted or filtered content
Webshops often use filters such as sorting by price or color. These filters can generate unique URLs, such as:
- https://www.yoursite.com/clothing?sort=price-increasing
- https://www.yoursite.com/clothing?color=black
These pages can all be given a canonical tag to the main page:
<link rel="canonical" href="https://www.yoursite.com/clothing" />Important tips for using canonical tags
- Use absolute URLs, so with
https://and domain name - Make sure the page you are referring to exists and is reachable
- Only use one canonical tag per page
- Do not use it to pages; that is counterproductive
- Preferably use it in combination with good internal linking
How do you add a canonical tag in HTML?
Place the following line in the <head> of your HTML document:
<head>
<link rel="canonical" href="https://www.yoursite.com/page" />
</head>What if you use a CMS?
In WordPress, you can use plugins such as Yoast SEO. These automatically add canonical tags. Check whether they are set correctly, especially for categories, tags or custom URLs.
How to check if your canonical tag is working?
- Look in the
<head>of your page via the source code (right click → View page source) - Use tools like SEO Review Tools Canonical Checker
- Check in Google Search Console if the correct URL is indexed
Common mistakes
- Canonical tags refer to incorrect or non-existent URLs
- Each page refers to itself and to another page at the same time (conflict)
- Forgotten canonical tags on dynamic or filter pages
Conclusion
Canonical tags are a simple but essential technique in SEO. They help search engines understand which version of a page is important, and prevent your content from being seen as duplicate. By correctly applying canonical tags, you strengthen your position in search engines and maintain the value of your links.
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