Confirmation from the Authority

For years I've been hearing that you should use a hyphen (-) and not an underscore (_) in urls. I've never really understood why (Google really can't figure out that an underscore is a space????), and have never seen anyone give any proof.

But as I was researching something else in Google's help docs, I ran across this:

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.

So there you have it. Straight from the source - Google wants you to use a hyphen.

Here's the original - URL structure