// definition
Canonical URL
The page version you want Google to treat as the original among duplicates, declared with rel=canonical so ranking signals consolidate to one address.
What it is
A canonical URL is the version of a page you nominate as the "real" one when the same or very similar content lives at more than one address. You declare it with a <link rel="canonical" href="..."> tag in the page head (or an equivalent HTTP header).
Duplicates are more common than people expect: http and https, www and non-www, trailing slashes, tracking parameters like ?utm_source=, and printer-friendly versions all create separate URLs that serve the same content. The canonical tells Google which one to index and show in results.
Why it matters
Without a clear canonical, Google picks one version for you, and it may not be the one you wanted. Worse, link signals and relevance get split across the duplicates instead of pooling behind a single page.
A canonical fixes that by consolidating signals onto your preferred URL. To be clear: there is no duplicate-content penalty, just wasted crawl effort and diluted signals, per Google's SEO starter guide. Treat the tag as a strong hint, not a command, and back it up with consistent internal links and redirects pointing at the same address.
Canonicals are one of the foundational checks in our technical SEO checklist for a new site.
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