The horizontal form DIVIDER control is used to partition controls above and below it.The programmer binds other controls to the horizontal divider such that the user can resize individual sections of a form or FRAME by interactivly dragging the divider with the mouse while the form is being displayed.
Horizontal and vertical form DIVIDER controls are actually the same DIVIDER control. They are only differentiated by their height to width ratio. A DIVIDER that is higher than it is wide is treated as a vertical DIVIDER. A DIVIDER that is wider than it is high is treated as a horizontal DIVIDER. To change a DIVIDER from a horizontal DIVIDER to a vertical DIVIDER (and vica vera) simply change the shape of the DIVIDER by grabbing it at the corner of (while in edit mode) and dragging the corner to a new position.