1,3,5,7-Tetraoxadecalin has been introduced as a novel central unit into the chemistry of calamitic liquid crystals. 2e,6e-Disubstituted derivatives 3a−h and 4b, g, i of the trans or cis fused heterobicyclus, respectively, have been synthesized. Whereas most of these tram-products (3c−h) are nematic or smectic A (new “rod-like mesogens”), the cis-products (4b, g, i) are not liquid crystalline. The configurations and conformations of all compounds have been elucidated by spectroscopic methods in particular by 1 H and 13 C NMR. In addition the structures of 3d, 3g and 4i as representatives of both series have been confirmed by X-ray analysis. The 1,3,5,7- tetraoxadecalin unit in the non-mesomorphic ds-products exists, as expected, in the “O-inside" conformation which has been established both by 1 H NMR and X-ray studies. The relationship between the chemical structure and mesomorphic behaviour of these 1,3,5,7-tetraoxadecalin derivatives is discussed, as well as the most likely structural requirements for turning the non- mesomorphic cis-products into mesomorphic ones. The name “roof-like mesogens" is proposed for this predicted new type of liquid crystals with three-dimensional geometry.