Dash or Underscore? What to Use In URLs?

        Using dashes in URL is good compared to underscores. Using URL s properly would definitely contribute to make MY SITE RANKS FIRST IN GOOGLE

i.e. xxx-yyy site is fine when compared to xxx_yyy site. It improves page rank

Because xxx-yyy site could be returned for queries like

1)      xxx

2)      yyy

3)      xxxyyy

4)      xxx-yyy

But xxx_yyy could be returned if user exactly gives the query xxx_yyy.
use it properly and make MY SITE RANKS FIRST IN GOOGLE