Figure 31 Symmetry and shape of some unit cells corresponding with certain rotation axes (reflection planes are not shown): (a) four-fold rotation axis - crystallographic axes at 90° to each other, and the unit cell has a square cross-section; (b) single two-fold rotation axis - constrains two of the crystallographic axes to be at 90° to each other; (c) three two-fold rotation axes at 90° to one another - implies that three crystallographic axes are also at 90° to each other.