Canonical URL

The same content is often reachable at several addresses: with and without a trailing slash, with tracking parameters, over www and without, in a paginated series. Each is technically a separate page. Without a canonical, search engines split the page's value across variants and may index the wrong one. The canonical tag consolidates them.

A self-referencing canonical, where every page points to itself, is the baseline and should be on every indexable page. Pointing a variant at its master is the second case. Pointing a page at a completely different page is the third and usually a mistake, since it tells search engines to drop the page from the index.

Canonicals become important during migrations and redesigns, when old and new addresses coexist for a while. They belong in the launch checklist alongside redirects and the sitemap, and they should be verified after launch rather than assumed.