I haven't tried to copy child selecter info out of the browser, but here's how the work. Take this menu as an example:
<ul id="parent-menu">
<li><a>Menu Item #1</a>
<ul id="child-menu">
<li><a>Submenu Item #1</a></li>
<li><a>Submenu Item #2</a></li>
</ul>
<li><a>Menu Item #2</a></li>
</ul>
Your standard CSS without any selectors would look like this:
#parent-menu li{ width: 33%; }
The problem is, that applies equally to the list items in the parent menu and the list items in the submenu. The ">" selector means "direct descendant." It will apply to child elements, but not grandchild elements. For example:
#parent-menu > li{ width: 33%; }
This will not apply to submenu list elements, because they are't direct descendants of the parent menu (there are other elements in between). I hope that helps a little.
Now, to keep the arrows from wrapping, add the following property just below the width: 33%; we discussed earlier:
white-space: nowrap;