Hi. Canonical allows you to specify your preferred version of a URL. You would implement it if Google are indexing both www and non www. If that happens you need to 301 redirect the non preferred version to the preferred version (www and non www are treated as two different versions) the 301 redirect can be done by configuring your htaccess file that normally resides in the root of your domain (sites hosted on Apache server) the 301 redirect helps to make sure that link popularity is consolidated to your preferred version. For pages with similar content you can specify the canonical version (preferred version) by using rel="canonical" in the head section of both pages (they need to point to the preferred version) Always remembering that PageRank and other signals will also be transferred to the canonical page (the original article page) so from a SEO point of view it is very important to implement a 301 redirect (if Google index both versions) and to use rel="canonical" for similar pages. That would reduce the chance of your pages been filtered out of the search due to copy issues. Google also take rel="canonical" as a strong hint, taking your preference into account with other signals when determining the most relevant page to show in search results. It is also important that you update links to point to a single canonical page. That would ensure optimal canonicalization in the search results. Canonical can also be used via cross domains (different domains).