Symmetry of Crystal Lattices
One important use of point groups is the desription of the shapes of unit cells. In the table below are listed the seven crystal classes along with the restrictions on the cell parameters and the lattice symmetries:



*The trigonal sub-system is an exceptional case: the lattice of trigonal structures is hexagonal (symmetry 6/mmm) but the contents of the unit cell have only 3-fold symmetry.

The next page explains and justifies the symmetry labels in this table.