Provide one version of a URL to reach a document

To prevent users from linking to one version of a URL and others linking to a different version (this could split the reputation of that content between the URLs), focus on using and referring to one URL in the structure and internal linking of your pages. If you do find that people are accessing the same content through multiple URLs, setting up a 301 redirect from non-preferred URLs to the dominant URL is a good solution for this. You may also use canonical URL or use the rel="canonical" link element if you cannot redirect.
Avoid:
 having pages from subdomains and the root directory access the same content
- e.g. "domain.com/page.htm" and "sub.domain.com/page.htm" using odd capitalization of URLs
- many users expect lower-case URLs and remember them better

Choose a URL that will be easy for users and search engines to understand!