15.40.6  GRID 

Draws  grid lines corresponding to Z major ticks on a histogram of a MESH.
These are drawn according to the style,color...  selected by the  SET GRID
command.   The  CYCLE  option  in  the  SET  GRID  command  can make these
different colors and textures.
(Default:GRID=OFF)

If  SET  AXES  THREE=ON  and  SET  GRID  X=ON  Y=ON are selected then GRID
defaults to ON.