A number of Web search engines enable you to specify the canonical form of the URL for an indexed page. For example, suppose your site has a page that can be accessed by several different URLs (either because the query parameters can vary, or because there are multiple paths to the same page). Rather than indexing the page separately by each different URL (and diluting the page ranking as a result), you can instruct search engines to index the page by a single URL in its standard (canonical) form.

You specify the canonical URL for a page using a link tag with its rel attribute set to "canonical". For example:

<link rel="canonical" href="www.example.com/product/Blue+Suede+Shoes" />

When a web spider crawls a page, it records the page’s URL as the value specified in the href attribute, rather than the actual URL that was used to access the page.