about css style in gwt

in gwt developer guilde http://code.google.com/webtoolkit/doc/latest/DevGuideUiCss.html

complex style section:

 .gwt-MenuBar {         /* properties applying to the menu bar itself */     }    .gwt-MenuBar .gwt-MenuItem {         /* properties applying to the menu bar's menu items */     }    .gwt-MenuBar .gwt-MenuItem-selected {         /* properties applying to the menu bar's selected menu items */    }

I cannout underatand, why the css rule is

.gwt-MenuBar .gwt-MenuItem

but not

.gwt-MenuItem
?
I am not good in css, is
.gwt-MenuBar .gwt-MenuItem  is equal to just one css class? not any special meaning for the prefix .gwt-MenuBar, that is
we can rename it as 
.gwt-MenuBargwt-MenuItem will not any different? 

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