We have studied antiferroelectricity in twin dimers, in which two mesogens with alkyl tails are linked by an alkylene spacer. Since the conformation of dimers is confined by the spacer, the use of the spacer with an odd carbon number forces the dimer to assume a bent conformation. If a segregation between the spacer and tail takes place in such odd-numbered dimers, the smectic structure formed is biaxial with bent molecules packed into a layer with the same bent directionality. Due to this characteristic packing, the spontaneous polarization appears parallel to the layer even in an achiral system. According to this understanding, we have obtained the antiferroelectric smectic liquid crystal which shows the distinct tristable switching phenomenon with the application of a triangular voltage wave.