The alignment of multilamellar membranes is quantified by the distribution function of the membrane normal vectors, which are directed along the z axis perpendicular to the solid substrate on which the membranes are deposited. The central width of this distribution is called mosaicity. A narrow distribution or equivalently small mosaicity allows for a precise distinction between the scattering vector components, vertical (q z ) and parallel (q k ) to the lipid bilayer. Thereby, the multilamellar state of the samples, which can be almost fully hydrated by controlled relative humidity (Aeffner et al. 2009 ), provides a tremendous amplification of the scattering signal compared to single bilayer preparations. However, compared to single bilayer reflectivity, the multilamellar stacks usually impede the determination of the electron density q(z) on an absolute scale, unless a full q-range fit of the electron density is performed (Constantin et al. 2003) .