desktop order switching

Hi,

Does anyone know what controls the ordering of the Desktops/Spaces? The order keeps changing so when I go left or right with three fingers, the order I go through them changes. Now, for some reason they are ordered Desktop 1, 4, 2, 3. When I started it was 1, 2, 3, 4 but then 2 and 3 switched around. Now they are in the correct order, but 4 is 2nd for some reason. Does anyone know how they are moved so I can put them back in the correct order and how to keep them from moving?