Definition
A canonical URL is the preferred version of a page that you declare to search engines when the same content is reachable at multiple addresses. You set it with a link tag in the head: <link rel="canonical" href="https://example.com/page/">. Search engines use it to consolidate the signals of all variants onto one address and show that version in the results.
Duplicates appear faster than you might expect. The same page often exists with and without a trailing slash, with http and https, with and without www, with tracking parameters (?utm_source=…), through filters and sort orders in a webshop, and as a print version. To Google those are separate URLs. Without a canonical, the search engine spreads links and authority across those variants, picks a version itself, and wastes crawl budget on copies.
The canonical is a hint, not a command. Google usually follows it, but deviates when other signals contradict it: internal links pointing to a different version, a sitemap listing another URL, a redirect, or hreflang tags referencing the non-canonical version. Consistency is therefore crucial. Every signal — canonical, internal links, sitemap, hreflang, redirects — must point to the same URL.
Common mistakes include a canonical pointing to a 404 or a redirect, a relative instead of an absolute URL, a canonical to the homepage on every page (a classic CMS bug), and paginated series that all canonicalise to page 1, so that page 2 onwards never gets indexed. Every page should carry a self-referencing canonical unless it is deliberately a duplicate.
Canonicals also work across domains: if you syndicate content to a partner, that page can point a canonical to your original so you keep the ranking. Traze adds a self-referencing canonical to every page it publishes and checks during technical scans that canonicals, hreflang and the sitemap agree with each other.