Biological membranes contain ordered and disordered domains. Small cholesterol containing ordered domains are called rafts. Ordered domains from the two monolayers are always in register as has been shown using the somewhat larger-sized liquid-ordered domains in artificial lipid bilayers. somehow thicker ordered domains and the thinner disordered domains. The goal of the present application is to dissect the predictions of these hypotheses and thus to elucidate the molecular mechanism for lipid domain registration. We start by measuring the interaction energy between both leaflets. Therefore small ordered domains will be induced by polymer adsorption to one leaflet of free-standing planar bilayers and their slippage against the other leaflet will be measured by fluorescence correlation spectroscopy as a function of temperature. Both by fluorescence correlation spectroscopy and fluorescence imaging, the slippage of larger domains of raft forming lipids in one leaflet against non-raft lipids in both leaflets will be monitored and the dependency of the friction on different lipid species will be established. This enables us to separate whether, acting via overhang at the midplane, ordered domains in one leaflet may induce lipids in the other leaflet to adopt an ordered state or whether the macroscopic appearance of domains may be explained by line tension driven merger of invisibly small domains into larger ones. In addition, we will exploit both electrostriction and hydrostatic pressure to test the theoretical dependence of domain size on surface tension. To clarify the role of line tension we will use polymers to induce domains of different sizes in the two leaflets and we will measure how their interaction energy depends on the mismatch in their sizes. Last but not least we will...