If the current panel is a split container that is docked to the form/user control and the DockVertical property is set to true, the container's children will be arranged vertically. Otherwise, the container's child panels will be arranged horizontally.
The DockVertical property can be set for a split container, or for a panel which will be used to create a split container (this is the panel to which another panel is docked when a split container is created).
When a panel is floating, use the FloatVertical property to choose between the vertical and horizontal orientation of the panel's children.