This is a standard drafting table. The horizontal bar is a straight-edge which can be moved up and down by the user. The criss-cross pattern of the cables stabilizes the bar and keeps it horizontal.

This effect can be seen by following the direction of motion of the two cables which comprise the mechanism. Note that all of the vertical arrows point in the same direction.

You could imagine attaching a stepper motor to one of the pulleys. Now, the horizontal bar can be moved up and down under computer control. This might be called a single-axis CNC stage.

How might we modify this mechanism to convert it into a two-axis CNC stage? The illustrated mechanism above is one solution. Rotating both motors in the same direction results in horizontal motion. Rotating both motors in opposite directions results in vertical motion.