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The potential energy diagram is even more clear if we plot it as a surface with the depth of "hills and valleys" representing the gravitational potential. The gravitational potential energy for a binary system is plotted in the adjacent diagram. The stars are located at the positions of the two deep depressions, but we see that as the stars approach each other a saddle-shaped valley develops between them. The inner Lagrange point sits in this saddle. Mass can flow through this region between the two stars if they are close enough together, just as a ball placed in the center of such a valley could roll either into one depression or the other.