Abstract: Below the thermotropic chain melting transition lipid membrane cP traces display a transition of low enthalpy called the lipid pretransition. It is linked to the formation of periodic ripples on the membrane surface. In the literature these two transitions are usually regarded as independent events. Here, we present a model that is based on the assumption that both, pretransition and main transition, are caused by the same physical effect, namely chain melting. The splitting of the melting process into two peaks is found to be a consequence of the coupling of structural changes and chain melting events. On the basis of this concept we performed Monte-Carlo simulations using two coupled monolayer lattices. In this calculation ripples are considered to be one-dimensional defects of fluid lipid molecules. Since lipids change their area by about 24% upon melting, line defects are the only ones, which are topologically possible in a triangular lattice. The formation of a fluid line defect on one monolayer leads to a local bending of the membrane. Geometric constraints result in the formation of periodic patterns of gel and fluid domains. This model for the first time is able to predict heat capacity profiles, which are comparable to the experimental cP traces that we obtained using calorimetry. The basic assumptions are in agreement with a large number of experimental observations.