The dihedral group of order 8 (D4) is the smallest example of a group that is not a T-group. Any of its two Klein four-group subgroups (which are normal in D4) has as normal subgroup order-2 subgroups generated by a reflection (flip) in D4, but these subgroups are not normal in D4.
the dihedral group of order 8 (d4) is the smallest example of a group that is not a t-group any of its two klein four-group subgroups (which are normal in d4) has as normal subgroup order-2 subgroups generated by a reflection (flip) in d4 but these subgroups are not normal in d4