This is called as Hbot mechanism.
Hbot = H-shaped robot.
Linear motions of orange object along X and Y axes are controlled by pink and green driving pulleys.
One branch of the black timing belt is fixed to the orange block, the other branch goes through it.
When pink and green pulleys rotate in opposite directions at the same velocity, the object moves along X axis.
When pink and green pulleys rotate in the same direction at the same velocity, the object moves along Y axis.
The green line is trajectory of a point on the object.
The mechanism can be used for 3D printers.
When the pink pulley turns P deg. clockwise (the green one is immobile) the orange object displacements are:
Along X axis: ΔXp = - Pi.D.P/720          
Along Y axis: ΔYp = - Pi.D.P/360          
When the green pulley turns G deg. clockwise (the pink one is immobile) the orange object displacements are:
Along X axis: ΔXg =  Pi.D.G/720          
Along Y axis: ΔYg = - Pi.D.G/360          
Pi = 3.1416
D: pulley diameter.
Programming driving pulley…