Webmaster Level: Intermediate to Advanced
Including a rel=canonical link in your webpage is a strong hint to search engines your about preferred version to index among duplicate pages on the web. It’s supported by several search engines, including Yahoo!, Bing, and Google. The rel=canonical link consolidates indexing properties from the duplicates, like their inbound links, as well as specifies which URL you’d like displayed in search results. However, rel=canonical can be a bit tricky because it’s not very obvious when there’s a misconfiguration.
While the webmaster sees the “red velvet” page on the left in their browser, search engines notice on the webmaster’s unintended “blue velvet” rel=canonical on the right.