What is a canonical tag and when should you use one?

A canonical tag is a line in a page's <head> that tells search engines which URL is the main version of that content. Use it whenever the same or very similar content is reachable at more than one address, so Google combines the signals into one URL instead of splitting them across duplicates.

What does a canonical tag look like?

<link rel="canonical" href="https://www.example.com/shoes/running">

Use the full absolute URL, including https:// and the domain.

When do you need one?

Duplicate URLs appear on almost every site without anyone meaning to create them:

• Tracking parameters: /shoes?utm_source=newsletter
• Sorting and filtering: /shoes?sort=price
• http and https, or www and non-www versions
• The same product in several categories
• Printer-friendly or AMP versions

Give every indexable page a self-referencing canonical, pointing to its own clean URL. Then any duplicate version automatically points back to the main one.

Is a canonical tag a command?

No. Google treats it as a strong hint, not a directive. It may choose a different URL if your other signals disagree, for example if internal links, the sitemap and redirects all point somewhere else. Keep them consistent. You can see which URL Google chose with the URL Inspection tool in Search Console.

What are the most common canonical mistakes?

Canonical, redirect or noindex?

• Use a 301 redirect when the old URL should no longer be visited at all.
• Use a canonical when both URLs need to keep working for visitors.
• Use noindex when a page should not appear in search, and has no main version elsewhere.

Google covers this in How to specify a canonical URL.