The influence on the transition temperatures of a carbon–carbon double bond in the terminal alkenyloxy (ether) chain of four series of three-ring phenyl/biphenyl-pyrimidines has been systematically investigated. The position and configuration of the double bond has been varied systematically in order to determine the optimal configuration and conformation of the terminal chains for smectic C formation. Four positions of the two nitrogen atoms were chosen. This produced four isomeric series of pyrimidines and differences in the mesomorphic behaviour were observed. The dependence of the transition temperatures on chain length was studied for the same four isomeric pyrimidine series. Comparisons between the new three-ring phenyl/biphenyl-pyrimidines and the corresponding two-ring phenylpyrimidines revealed almost identical tendencies. These results are consistent with a linearly-extended conformation of the chain. Several of the new ethers exhibit remarkably low viscosity values (i.e. short response times) and wide switching angles in an optically active base mixture used for evaluation and comparison purposes. Hence they can be used to widen the temperature range of mixtures designed for electro-optic display devices based on ferroelectric effects (FLCDs) without leading to longer response times.