Use hyphens, not underscores, in file and folder names.
Search engines read a hyphen in a web page file name as a space.
- Example: Web-Standards-Design.html is read as Web Standards Design
The resulting set of words provides useful keywords for the search engine.
Conversely, search engines don’t do anything with underscores.
- Example: Web_Standards_Design.html is read as Web_Standards_Design
This one long word is not a useful keyword for the search engine, and won’t do anything to boost your web page’s page rank.
Likewise, use hyphens in URLs.
- Example: http://www.example.org/Web-Standards-Design/index.html. The folder name is read as Web Standards Design
Use file and folder names that are keyword rich.
From the book Building Findable Websites by Aarron Walter:
Separate keywords in file and folder names with a hyphen rather than an underscore to ensure that search engines can read each word individually rather than as one large word. For example, most search engines will read a file named my-page.html as “my” “page,” whereas my_page.html would be read as “my_page,” which is not likely to match a search query. Google recently updated its system to recognize individual words in file names separated by an underscore. Since other search engines could be tripped up by the underscore, stick with hyphen-delimited keywords in your file and folder names to be safe.