Ajax TabContainer - UseVerticalStripPlacement="true"

Hi All

Can anyone please throw any light on to why my tabContainer is still displaying horizontally?

<asp:tabcontainer runat="server" OnClientActiveTabChanged="ActiveTabChanged" 
                    ID="tabContainer" ActiveTabIndex="0"  
                    UseVerticalStripPlacement="left" VerticalStripWidth="120px">

I need the tabs to be on the left and vertical?