canonical: the one true URL

Why canonical exists

The same content can live at multiple URLs without you meaning it to:

• example.com/post vs example.com/post/ (trailing slash)
• https://example.com/post vs https://www.example.com/post
• /post vs /post?ref=twitter&utm_campaign=launch
• /post vs /p/123 (alias)

Google sees them as separate pages. PageRank gets split. Each version competes against the others. The fix:

<link rel="canonical" href="https://example.com/post" />

Tells Google "if you found this content at any other URL, treat THIS one as the master copy. Consolidate all rankings here."

Rules

• Absolute URL. /post doesn't work — must be full https://....
• Self-referential is fine. /post can have <link rel="canonical" href="https://example.com/post" /> pointing at itself. Even recommended.
• Cross-domain works. Republished content can canonical-link back to the original.
• Match the path you want indexed. If your CMS generates ?ref=twitter variants, the canonical should be the clean URL.

hreflang vs canonical

Confusing pair:

• canonical says "this URL is the master version of these duplicates."
• hreflang says "this is the EN version; the FR translation lives at /fr/post."

A multi-language page has BOTH: canonical to itself (the EN URL), plus hreflang siblings to its translations.