Transition temperatures of four twin types of troponoid liquid crystals, in which the direction of the tropone carbonyl groups and a kind of the spacer were different from each other, were discussed. Symmetrical twin dimers 1, in which two tropone carbonyl groups direct inside, had monotropic smectic C phases whereas another symmetrical twin dimers 3 were less mesogenic than 1. Unsymmetrical twin dimers 4 had smectic A phases. Compared their thermal stabilities, unsymmetrical 4 had the highest clearing point when the number of atoms of the inner spacer was identical.