URLs: Dash vs. Underscore
Here are a few points in favor of the dashes:
- Dashes are recommended by Google over underscores (source).
- Dashes are more familiar to the end user.
- Dashes are easier to write on a standard keyboard (no need to Shift).
- Dashes don't hide behind underlines.
- Dashes feel more native in the context of URLs as they are allowed in domain names.
It's not just dash vs. underscore:
- text with spaces
- textwithoutspaces
- encoded%20spaces%20in%20URL
- underscore_means_space
- dash-means-space
- plus+means+space
- camelCase
- PascalCase
- " quoted text with spaces" (and single quote vs. double quote)
- slash/means/space
- dot.means.space
From Google Webmaster Central
Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.