canonical tag
Also called: rel=canonical, canonical link element, canonical URL
A canonical tag, written as a rel=canonical link element in a page's HTML head, tells search engines which URL is the preferred version among duplicate or near-duplicate pages. Google treats it as a strong signal (not a strict rule) and uses it to consolidate ranking signals onto that one URL.
Duplicate or near-duplicate pages are normal. The same product can be reachable at ?color=blue, a print version, an HTTP and an HTTPS copy, or a URL with tracking parameters. Left alone, search engines have to guess which one to index, and link signals get split across the copies. A canonical tag points them all at a single preferred address.
How Google actually treats it
Google calls the canonical the “most representative” URL from a set of duplicates, and it picks one whether or not you declare a preference. Your rel=canonical is a strong hint, not a command. Google weighs it against redirects, HTTPS, sitemap inclusion, and internal links, and it can still choose a different URL if your signals conflict. The canonical page then gets crawled most often, while the duplicates are crawled less.
The syntax lives in the <head>:
<link rel="canonical" href="https://example.com/product" />Most pages should carry a self-referencing canonical that points to their own clean URL. Common mistakes: canonicalizing every page to the homepage, pointing to a noindex or redirected URL, or placing more than one canonical on a page. Each of those sends a conflicting signal and usually gets ignored.
How it affects your traffic
When canonicals are wrong, Google splits authority across duplicate URLs or indexes a version you did not want, which caps rankings for the pages that should be competing. On large sites, faceted filters and URL parameters can spawn thousands of near-duplicates that waste crawl budget and dilute link equity. Getting canonicalization right is a core part of a technical SEO audit. It also determines which single URL is eligible to surface in Google-index-derived experiences like AI Overviews, since Google indexes only the canonical from each duplicate set.
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