What is Canonical tag?

Why it matters

Almost every site serves the same content at more than one URL without meaning to: with and without a trailing slash, with tracking parameters attached, through two category paths, or in a print view. Left alone, search engines pick one themselves and the choice is often not the one you would have made.

How to use it

• Every indexable page carries a self-referential canonical pointing at its own preferred URL
• Use absolute URLs, not relative ones
• Filtered and sorted views canonicalise to the clean category URL
• Never canonicalise a page to something that is not genuinely equivalent — Google ignores it and you have wasted the signal
• A canonicalised URL should not also appear in your XML sitemap

The mistake people make

Canonicalising every page to the home page, usually by accident through a template variable that was never set per page. The result is that nothing else on the site is treated as worth indexing, and it can take months to notice because the pages still exist and still respond.

Where this is covered in depth

A definition can only go so far. Technical SEO, in the order the problems actually block you covers this properly — 4 minutes, free, no email required.

Who wrote this

Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.