Canonical URLs: one page, many addresses

The same content often lives at several URLs — with and without tracking parameters, with and without a trailing slash, printable and not. Canonicalisation tells a crawler which address is the real one.

The <link rel="canonical"> tag in a page's head names the preferred URL. When a crawler fetches a duplicate that points its canonical at this page, it is asked to consolidate — to credit and index the original, not the copy. There is a duplicate of this very page; open it and read its head.

Get it wrong and you split your signals across near-identical pages, or worse, canonicalise everything to one URL and disappear the rest. It is a small tag with an outsized effect on what a machine believes your site contains.