By adding rel="canonical" to your pages you can tell search engines about the preferred content / page for a given page or set of pages ....
<link rel="canonical" href="http://xmlsitemapgenerator.org"/>
Why use it?The rel="canonical" element helps search engines reduce duplicate content. To help de-duplicate your content and prevent yourself from being seen to publish duplicate content you should include a canonical url for similar/the same pages.
When to use it
- Sites with dynamic URLs which can lead to the same content
- eCommerce sites, especially on product listings where the same content may be displayed in various sort orders or products with multiple pages for minor variations (e.g. colour)
- Syndicated content to point to the original content.
When not to use it
- If your page has moved or there is a preferred version your first choice should be a HTTP 301 redirect. This not only tells the search engine the correct page, but will also redirect the user to the correct version of the page.
- This includes if you are changing your website structure, use HTTP 301 redirects.