Canonical tags, explained simply

Websites generate duplicate and near-duplicate URLs constantly - tracking parameters, print versions, http and https, www and non-www, faceted filters. Left alone, these compete with each other and split the authority that should accrue to one page. The canonical tag resolves the conflict by declaring: 'of this set, treat this URL as the original.' Engines then consolidate signals onto that URL and treat the rest as alternates.

The rules are simple, and most canonical problems come from breaking one of them.

These three tools solve different problems and shouldn't be confused. Use a canonical when two pages should both exist but one is the master for ranking. Use noindex when a page should be crawlable but never appear in results. Use a 301 redirect when a page has genuinely moved and the old URL should cease to exist. Reaching for the wrong one - say, canonicalizing pages that should redirect - leaves engines guessing.

Because the canonical tells an engine which URL to keep, an incorrect one can silently remove a page from results: if page A wrongly canonicalizes to page B, A may never be indexed on its own. It's a common, hard-to-spot mistake - the page looks fine, ranks for nothing, and the cause is one line in the head. The same applies to AI citation: an engine consolidates onto the canonical, so point it at the version you actually want quoted.