I just read this interesting article by the rather famous Mr Cutts last week and have been passing it onto my clients for future reference. Rather than lose it to the vagaries of my failing memory 
I have decided to add it to my blog for you to read, learn and enjoy…
Dashes vs. underscores
I often get asked whether I’d recommend dashes or underscores for words in urls. For urls in Google, I would recommend using dashes. Why? To find out, let’s take a trip in the Google Time Machine. Set the dial for 1999, the year Matt first discovered Google. Matt was using, I dunno, maybe HotBot at that point? The curtain rises:
Matt: Hmm, this search for [FTP_BINARY] didn’t turn out the way I wanted. I got a couple scuzzy looking urls, and the other documents just have the words “FTP” and “BINARY” but the term “FTP_BINARY” doesn’t actually appear. (Note: Matt was a bit of a nerd, as you can tell.)
Some Random Person That I Don’t Remember: Have you tried Google?
Matt: What’s that?
SROTIDR: It’s a search engine written by nerds for nerds! They index numbers! Sometimes they even index punctuation, like “C++”. Try your underscore search there.
Matt: Okay, here goes. Whoa! They actually return pages with the literal string “FTP_BINARY”! That’s wicked cool! (Did I mention Matt was a nerd? Big-time nerd.)
SROTIDR: Yeah. The wild thing is that they wrote a paper about how they crawl the web and rank pages.
Matt: Well, now that’s just silly. I wonder why they didn’t keep it a secret? I bet those papers will make great reading for my information retrieval class.I’ve stylized the conversation quite a bit, but I remember how impressed I was that Google indexed numbers and some punctuation (come to think of it, search engines have come a long way in five years). With underscores, Google’s programmer roots are showing. Lots of computer programming languages have stuff like _MAXINT, which may be different than MAXINT. So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). If you have a url like word1-word2, that page can be returned for the searches word1, word2, and even “word1 word2″.
Some Random Person That I Don’t Remember: Have you tried Google?
Matt: What’s that?
SROTIDR: It’s a search engine written by nerds for nerds! They index numbers! Sometimes they even index punctuation, like “C++”. Try your underscore search there.
Matt: Okay, here goes. Whoa! They actually return pages with the literal string “FTP_BINARY”! That’s wicked cool! (Did I mention Matt was a nerd? Big-time nerd.)
SROTIDR: Yeah. The wild thing is that they wrote a paper about how they crawl the web and rank pages.
Matt: Well, now that’s just silly. I wonder why they didn’t keep it a secret? I bet those papers will make great reading for my information retrieval class.I’ve stylized the conversation quite a bit, but I remember how impressed I was that Google indexed numbers and some punctuation (come to think of it, search engines have come a long way in five years). With underscores, Google’s programmer roots are showing. Lots of computer programming languages have stuff like _MAXINT, which may be different than MAXINT. So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). If you have a url like word1-word2, that page can be returned for the searches word1, word2, and even “word1 word2″.
That’s why I would always choose dashes instead of underscores. To answer a common question, Google doesn’t algorithmically penalize for dashes in the url. Of course I can only speak for Google, not other search engines. And bear in mind that if your domain looks like www.buy-cheap-viagra-online-while-consolidating-your-debt-so-you-can-play-texas-holdem-while-watching-porn.com, that may still attract attention for other reasons.
