Canonical Tag
A tag telling the engine which URL is the original when several serve the same content.
Analogy Mental model
The copyright page. However many shops stock the book, one publisher and one ISBN are printed inside it.
In practice What you actually do
This is how faceted ecommerce survives: colour, size and sort order generate thousands of URLs, and a canonical on each points back to the clean product page. The same mechanism settles http versus https and www versus bare domain, and credits the original when content is syndicated.
Where people get burned Pitfalls
It is a hint, not an instruction. When two URLs differ substantially, the engine may ignore it and choose for itself. Never canonicalise page two of a paginated series back to page one, because that removes the deeper pages from the index entirely.
This entry was written in English from the Chinese original, not machine-translated. Read the Chinese version →