A straight line motion mechanism is formed from a pair of parallelogram linkage mechanisms in combination with motion control means interconnecting the linkage mechanisms to provide for uniform angular displacement thereof. The mechanism is particularly adapted for use in supporting an object, such as a table top, for vertically directed stepwise movement or one step shifting between lower and upper reference positions corresponding to table top heights convenient for use alternatively by a person in seated and standing positions. In a preferred embodiment, means are provided to enable a user to selectively make fine adjustments of the height of the table top after it has been shifted into either of such lower and upper reference positions.