Canonicalization
Choosing and declaring the official URL among several showing the same content.
Analogy Mental model
Deciding which is the authorised edition. However many copies circulate, one is the one the publisher recognises.
In practice What you actually do
Every page declares a canonical, including pages that are already unique, so the declaration becomes a habit rather than an emergency. Cross-domain canonical when you syndicate, and never point one at a URL that is itself noindex.
Where people get burned Pitfalls
The tag is a hint, and a large content difference gives the engine a reason to overrule it. Two pages differing by a paragraph of introduction are not duplicates, and treating them as such discards a page that might have ranked.
This entry was written in English from the Chinese original, not machine-translated. Read the Chinese version →