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A canonical URL is the preferred version of a page when multiple URLs return identical or very similar content. You signal canonicalisation with a <link rel='canonical' href='...'> tag in the page head. Use canonicals to prevent duplicate content penalties when the same page is accessible via multiple URLs (e.g. with/without www, with/without trailing slash, or via different filter parameters).
The most common mistakes are: canonical pointing to a different domain (unintentional), self-referencing canonicals on paginated pages (preventing deeper pages from indexing), canonical pointing to a noindexed page, canonical chain (A→B→C instead of A→C), and missing canonicals on pages accessible via multiple URL variations. All of these dilute link equity and cause indexation issues.
Yes. A canonical tag passes most of the link equity (PageRank) from the non-canonical version to the canonical URL — similar to a 301 redirect but without the browser redirect. This consolidates ranking signals from duplicate URLs into the preferred version, improving its overall ranking authority.
A 301 redirect sends both users and bots to a different URL immediately — the old URL disappears. A canonical tag leaves both URLs accessible but signals which one Google should index. Use 301 redirects for permanently moved pages. Use canonical tags for duplicate content variants (e.g. tracking parameters, session IDs) where you want both URLs to remain technically accessible.