Figure 1618a shows that, for a three-fold symmetry, each atom is surrounded by three identical atoms in a repeating pattern. The same pattern is produced if the image is rotated 120°.

Figure 1618a. Three-fold symmetry.

Figure 1618b (a) shows a three-fold rotation axis in crystallography, and Figure 1618 (b) shows the two-fold, three-fold and four-fold axes in a cubic structure.

Figure 1618b. (a) A three-fold axis, and (b) The two-fold, three-fold and four-fold axes in a cubic structure.