You can customize the way tabs are rendered by setting the Orientation property of RadTabStrip. Accepted values are:

HorizontalTop:

RadTabStrip is above the content (e.g. RadMultiPage). Child tabs (if any) are shown below parent tabs.

HorizontalBottom

RadTabStrip is below the content (e.g. RadMultiPage). Child tabs (if any) are shown above parent tabs.

VerticalRight:

RadTabStrip is on the right side of the content (e.g. RadMultiPage). Child tabs (if any) are shown on the left side of parent tabs.

VerticalLeft:

RadTabStrip is on the left side of the content (e.g. RadMultiPage). Child tabs (if any) are shown on the right side of parent tabs.