For some time the seo specialists (and a lot of other guys that have some idea about search engine optimization) has recommended that URLs on the page should contain hyphens instead underscore because the some search engines were considering hyphens as word separators but the underscores were just letters inside words. But lately, at least since 2007 for Google, all main search engines are considering both as word separators. Some of them considered underscores as separators from the beginning. There are people that still don’t know that: I’ve seen recent articles (written at the end of last year) that still treats underscores as letter inside words.
As a web developer I started to use underscores to separate words from the beginning because of the language: I am generating the URL from the title or the article (whatever the content – no matter if it’s a blog, a news portal or an online store) and there are a lot of words that uses hyphens as the grammar requires in the Romanian language (I could give you some examples, but it would be meaningless for the most of you) and even some names contains hyphens.
As I told you in the earlier articles, the best URL is the one that’s readable by both the humans and the search engines and it’s the clearest and easiest to understand. From my point of view, these conditions are accomplished using underscores as word separators and the hyphens only where they are needed.
WordPress, for example, started from the beginning with the hyphens as word deliminator and they are stick with it. That’s a good thing, even sometimes I am not sure if the names in the titles are of two individuals or one name created from two other names put together.
The source is here.