What are Canonical URLs and why use them?
When talking in geek speak, canonicalization is a way to normalize data that can be represented in multiple ways, so that it can be addressed in one standardized format.
URL canonicalization is used when a page can be viewed at more than one URL. Specifying one “main” canonical URL can help when there are different versions of the same page for screen view and printing, or different version of a page for desktop and mobile, or a content management system creates the same page but with different URLs, or there may be variables that appear in the URL. Examples include:
In these cases, if a canonical URL is not set, search engines will decide for themselves which URL to display. “Declaring” the canonical URL gives you control over which URL a search engine uses to display your page in a search result page.
In short, specifying a canonical URL is a good idea to maintain control over which URLs appear in search results.
Final note: setting a canonical URL has no effect on your page’s search ranking, but does affect which page(s) appear in search.