The dihedral rotation group is defined by having  a C₂ axis perpendicular to the principal C_n axis.  In fact, there will be nC₂'s perpendicular to the C_n, although finding only one is sufficient to classify the molecule as dihedral.  This rotation group can be further subdivided into 3 point groups: D_n, D_nh and D_nd.

• D_n  -  The absence of a s_h and a s_d (perpendicular and dihedral mirror planes), puts a molecule in this group.  Molecules in this point group will be chiral and will have no dipole moment.  An example is Co(en)₃.
• D_nh  -  This group is defined by the presence of a s_h, a mirror plane perpendicular to the principle C_n.  An example is BBr₃.
• D_nd  -  This group is defined by the presence of a s_d, a mirror plane bisecting two adjacent C₂'s.  For odd values of n, an inversion center will also be present, but is not used to assign the point group.  An example is ferrocene with staggered cyclopentadienyl rings.