sentences:  The families of groups with these symmetries are characterized by 2-fold proper rotation axes normal to a principal rotation axis.The dihedral groups are denoted by "D"n. These groups are characterized by i) an "n"-fold proper rotation axis "C"n; ii) "n" 2-fold proper rotation axes "C"2 normal to "C"n. The "D"1 group is the same as the "C"2 group in the cyclic groups section.The prismatic groups are denoted by "D"nh.