This question comes up occasionally when building a website and wanting to design it for absolutely optimal SEO, and it relates to how the URLs should be structured.
Using keywords in URLs is helpful for promoting that term, so if you wanted to promote your site for a search term like SEO in Adelaide then an ideal URL for a target page or directory would be: www.<your domain>/seo in adelaide
Unfortunately this will display using the hex charcaters for a space (2o hex) and look this this: /seo%20in%20adelaide
So to improve the display of this URL we can join the words with a dash (seo-in-adelaide) or an underscore (seo_in_adelaide) but which should you use for optimal SEO outcomes ?
Google’s Matt Cutts explains this including the history behind it here: