Individual words within your URL should be separated with hyphens so they can be easily recognised by search engines such as Google.
Search engines treat hyphens (–) and underscores (_) as spaces: http://www.nomensa.com/web–accessibility.html.(nomensa)
Use your keywords in your URL’s that relate to the content where appropriate.
Twitter set up by ex–Google employeers realised this early
But Facebook have only recently has realized how pretty urls are useful and they launched Facebook vanity URL’s.
Nomensa Article SEO tips
dated: September 2009
By Emily Coward
Links
WebpageSEO tips
Net tuts Search Optimization 101
dated: September 2009
By Siddarth
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WebpageSEO 101