Structure of a lipid monolayer can be characterized by its spontaneous shape corresponding to a stress-free state. In the chapter, the rigorous description of monolayer shape in terms of curvature of a Gibbs dividing surface is reviewed, and the notions of the neutral and pivotal surfaces are discussed. Further, the effective and exact method of finding the position of the pivotal surface and the corresponding monolayer spontaneous curvature of cylindrically curved monolayers based on the results of the X-ray scattering experiments are presented. Briefly, some limitations of this method are discussed.
