What is a canonical tag, and when do I need one?

What is a canonical tag?

A canonical tag is a link tag in a page's head with rel="canonical" and an href that holds the page's preferred address. It tells search engines which URL is the main version when the same, or nearly the same, content can be reached at several addresses. That happens more often than you would expect: with and without www, http and https, with and without a trailing slash, and with tracking or filter parameters such as ?utm_source=newsletter or ?sort=price.

Without a signal, Google picks one of those addresses itself, and your links and visits can be split across several versions. The canonical tag points everything at the one you chose.

When do you need a canonical tag?

Give every page you want indexed a self-referencing canonical, meaning one that points to its own preferred address. It costs nothing, protects you from parameter and tracking variants, and states your intent even when no duplicates exist yet. It matters most in these cases:

• Your site answers at more than one address, such as www and non-www or http and https. Redirect the extras and set canonicals to the version you chose.
• Tracking, sorting or filter parameters create many URLs for one page.
• The same product or article appears at different URLs in several categories.
• A partner republishes your article and agrees to point its canonical at your original.

Is the canonical tag a command?

No. It is a strong hint. Google weighs several signals when it chooses a canonical: the tag, redirects, whether the address is in your sitemap, whether it uses https, and how your own pages link to each other. If they disagree, for example the tag names one address while every internal link and the sitemap use another, Google may choose differently and ignore your tag.

If an old address should disappear, use a 301 redirect, which is a stronger signal. Use a canonical when both addresses must stay reachable, such as a page with a sorting option. For the bigger picture, read whether duplicate content hurts SEO.

What are the most common canonical mistakes?

• Pointing the canonical at an address that redirects, returns an error or is blocked. Point it at the final, live address.
• Combining noindex with a canonical that points to a different page. The two send opposite messages, treat that other page as the main one and keep this page out, so Google may ignore one. Use noindex to remove a page and a canonical to merge duplicates.
• Mixed versions: an http canonical on an https site, or a canonical to a staging address left over from a migration.
• Pointing every canonical at the home page. Google tends to ignore canonicals that point to unrelated content.
• Two canonical tags, from a theme and a plugin, that disagree. Google ignores them, and it also ignores a canonical placed in the body instead of the head.
• Canonicalizing page 2 of a paginated list to page 1, which hides the items listed on page 2.

How do you check your canonical tags?

1. Open the page, view its source and search for canonical. You should find exactly one, in the head, with the full address including https.
2. In Search Console, inspect the address with the URL Inspection tool. It shows the user-declared canonical (yours) and the Google-selected canonical. If they differ, work out why.
3. In the Page indexing report, “Alternate page with proper canonical tag” is normal for duplicates, while “Duplicate, Google chose different canonical than user” deserves a look.
4. Visit the address in the tag. It must load the page normally, without a redirect.

Arcline flags pages with no canonical, and canonicals that point off your site, to a broken page or to a different page, on every scan. Where one is missing, the autopilot (Premium) can add a canonical that points to the page itself. Changing an existing preferred address is a sensitive change that always waits for your approval. See what Arcline fixes.

Canonical, redirect or noindex: which one?