The old hyphen – or underscore _ question again
An existing client is planning a site redesign for next spring and has been asking me quite a lot of questions about the best way to change the site, she asked the old question about if hyphens or underscores are better word separators in the page name of a url. I automatically said hyphens are best for a new site, but underscores are OK if the site already uses them. Then I thought about it for a moment and said I would double check, having not looked into it for a few years, and wanted to be 100% sure that I was still offering the right advice, fortunately I found this video made this year from Googles Matt Cutts on this exact topic:
In the video he explains exactly why this is the case. Essentially Google sees an underscore and hyphen slightly differently in that a hyphen is seen as a space and therefore splits the words, so “undescore-or-hyphens” is seen as “underscore or hyphens”, whereas an underscore is essentially ignored and joins the words together so “underscore_or_hyphens” becomes “underscoreorhyphen”.
Therefore if you are creating a brand new site, I would always use the hyphen. That being said, I have a site which is probably pushing 10 years old now, it uses underscores throughout and is ranking very well, so although it must be an aspect of the ranking algorithm which Google uses to rank pages, it’s probably a very minor one.
So in the case of a site redesign you are likely to lose more by redirecting old pages with underscores, to new versions using hyphens than you would to just keep the url’s the same, especially if those pages have a lot of links pointing to them.