Figs. and show the twins in B 6 S and B 6 Se, respectively. These twins are different from those shown in Figs. and. It is not possible to identify these structures in the a,b,c axes, but it is possible in the a 1 ,b 1 ,c 1 axes. The matrix and twin reflections shown in the insets are next to each other. In Fig. , arrows indicate the reflections corresponding to one of the crystal lattices. In Fig. , the reflections of one of the lattices are indicated by circles. In both cases, the twinning planes in the a 1 ,b 1 ,c 1 axes are {202}, which correspond to (001) planes in the a,b,c axes. Thus, it is possible to state that the twinning in B 6 S and B 6 Se can be realized in two planes: (101) and (001) in the a,b,c axes or, correspondingly, (001) and (202) in the a 1 ,b 1 ,c 1 axes. The existence of two twinning planes in the materials studied is not surprising. For example, many metals with hcp structure have two twinning planes each. There are materials where the number of twinning planes can reach four .