What it looks like
A link element with rel="canonical" pointing at the preferred URL, usually the page itself. Every page that can be reached by more than one URL should carry one: with and without www, with tracking parameters, HTTP and HTTPS variants, and paginated or filtered copies.
The three ways it breaks
- Missing: duplicates float without a preferred version, so engines pick one, sometimes the wrong one.
- Pointing elsewhere: the page tells engines to index a different URL, so this one never ranks. Correct for a genuine duplicate, fatal on a page meant to stand alone.
- Contradicting noindex: the page says index me here and keep me out at the same time, usually because a plugin added one directive and a theme added the other.
How to check yours
View source and search for rel="canonical", then ask three questions: is it there, does it name the URL you want indexed, and does nothing else on the page contradict it. The canonical check below does exactly that from outside your network, including the redirect chain and the noindex contradiction.
- Canonical checkSelf-reference, contradictions and chains, tested live.
- Fix a missing canonicalWhat to do when the tag is absent or names the wrong URL.
- Why a page is not indexedThe six reasons in order, canonical among them.
- Redirect checkChains and loops that interact with canonicals.
- Canonical tags, in depthWhich version of your pages should exist.