Audit canonical tags of a URL, self-canonical, canonical chain, HTML vs HTTP header conflict, loop detection. The silent #1 cause of indexing problems.
It tells Google: "among several URLs that display the same content, here is the preferred URL to index." Typical cases: parameter variants (?utm_source=...), www / non-www, HTTP / HTTPS, trailing slash or not, tag or category archives that contain the same article. Without canonical, Google chooses on its own, and its choice may differ from yours.
A page that declares itself as canonical (<link rel="canonical" href="THIS_PAGE_URL">) is the recommended pattern for every indexable page. It avoids ambiguity when your URL is fetched with parameters (utm, fbclid, gclid). Without self-canonical, Google may pick any parametrized variant as the canonical.
If A → B → C, Google may stop at the first hop and index B instead of C. The official Google rule: the canonical must point directly to the final version, with no intermediary. A chain often reveals a bug, a 301 redirect plus a poorly configured canonical, or an inherited template that was not updated.
On conflict (the <link> HTML tag points to X, the Link: ...; rel="canonical" header points to Y), Google keeps the HTML head value. But this divergence is a sign of broken configuration, typically a CDN or middleware injecting a different canonical from the one generated by your CMS. To resolve at the infra layer.
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