The interesting thing is we used to treat underscores as if they were like word A underscore word B, we would glom that together and we would index that as A underscore B, so if you just searched for the word A, we wouldn’t return your post. Ah… We’re in the process of changing that. We might have already changed that. So dashes and underscores are almost exactly the same.You can hear the above statement for yourself in this video of Matt’s talk, at around the 17 minute mark. I excitedly wrote about it in a post for the News.com Blog, since historically keywords separated by underscores didn’t look like separate words to Google, and this would save a lot of folks a lot of time if they were embarking on a URL rewriting project to fix their underscore problem. Unfortunately I jumped the gun a bit, because Google still has not made the switch to recognizing underscores as word separators like they do with hyphens. Your next question might be “But are you sure??” Yup. When I spoke to Matt in February at SMX West, he confirmed that underscores were NOT treated as word separators. According to Matt, this change is still in their queue but unlikely to happen before summer. My interpretation: don’t hold your breath, it’s between summer and never. 😉 Why didn’t they roll out that change? Certainly it’s clear it’s not a priority. Google engineers are focused on improving relevancy and improving the searcher’s user experience. I would guess that this particular tweak to their algorithm isn’t going to do much for their users. So, in your URLs, keep favoring hyphens over underscores for the foreseeable future. And here’s one gotcha to be aware of: don’t use an underscore to separate a lookup ID from hyphenated keywords. For example, a URL like http://www.example.com/1234_nike-pegasus-running-shoes.html may at first glance appear to be search engine optimal, but the keyword “nike” is not visible to Google as a separate word. The keyword is actually understood by Google to be “1234_nike”, not “nike”. By the way, although I favor the hyphen, there are other word separators accepted by Google, such as the dot (.), the plus sign (+), and the “escaped” space character (%20).