Glossary
Canonical URL
A canonical URL is the preferred URL for a piece of content, declared via the rel='canonical' tag — used to prevent duplicate-content issues when the same content is accessible at multiple URLs.
Canonical URLs solve a common problem: the same content reachable at multiple URLs. Common causes: tracking parameters (?utm_source=twitter), trailing slashes (/page vs /page/), case variations (/page vs /Page), HTTP vs HTTPS, www vs apex, and pagination variants. Without a canonical signal, Google may treat each variant as a separate URL and split ranking signals across them.
The mechanism: a <link rel='canonical' href='https://example.com/page'> tag in the HTML head declares the preferred URL. Search engines (Googlebot, Bingbot) consolidate all duplicate variants onto that canonical URL — link equity, backlinks, indexing all flow to the canonical.
Self-referencing canonicals are best practice. Even on the 'main' URL of a piece of content, include a canonical pointing at itself. It prevents weird parameter combinations or proxy URLs from being treated as the canonical by accident.
Cross-domain canonicals are valid. If the same article appears on Medium and your own site, you can canonicalize Medium's copy back to your own URL — signals that your site is the authoritative source. Many high-volume blogs use this pattern to syndicate without losing SEO credit.
Common canonical mistakes: pointing canonical at the wrong URL (e.g., always pointing at the homepage), canonicalizing paginated archives onto the first page (Google deprecated this guidance — pagination should self-canonicalize), missing canonicals on programmatic pages (every variant looks duplicate to Google), and conflicting signals (canonical says one URL, redirect says another, internal links say a third).
Example
A blog post lives at /blog/seo-for-ai-sites. The same content is reachable at /blog/seo-for-ai-sites?utm_source=newsletter and /blog/seo-for-ai-sites/. All three variants include <link rel='canonical' href='https://example.com/blog/seo-for-ai-sites'>, consolidating ranking signals onto the clean URL.