The partial substitution of silicon by germanium in zeolite synthesis mixtures is well known to direct zeolite synthesis towards double-four ring (D4R) containing frameworks. Whereas the structure-directing effect of Ge is well described, the chemistry behind the effect is less well understood. In this study, we show that the structure-directing effect is strongly linked to the lowering of the energetic destabilisation of D4Rs compared to large, large-faced, cages, and more in particular that low Ge-substitution increases the probability of synthesizing frameworks containing D4Rs with respect to all other cage types on thermodynamic grounds. Furthermore, we demonstrate that the symmetry lowering of D4Rs with Ge-substitution, predicted on geometric grounds by O'Keeffe et al., can be rationalised in terms of the energy released by symmetry lowering. (C) 2004 Elsevier Inc. All rights reserved.