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Figure 2
The shape of the unit cells in different crystal systems. The unit-cell edges are in general a, b, c, but when there are relations of equality among them, this is indicated by repeating the same symbol. For instance, in the cubic system, the three edges are of equal length (and mutually perpendicular). The rhombohedral cell is a special case of a trigonal cell. It has a threefold axis along its body diagonal (extended direction); therefore, the three unit-cell edges must have the same length and the three unit-cell angles must be equal.

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