A point group is a group of symmetry operations which all leave at least one point unmoved. Although an isolated object may have an arbitrary Schönflies symbol, the requirement that symmetry be present in a lattice requires that only 1, 2, 3, 4, and 6-fold symmetry axes are possible (the crystallography restriction), which restricts the number of possible so-called crystallographic point groups to 32.

Cotton, F. A. "Character Tables for the Chemically Important Symmetry Groups." Appendix IIA in Chemical Applications of Group Theory, 3rd ed. New York: Wiley, pp. 426-436, 1990.

Hahn, T. (Ed.). International Tables for Crystallography, Vol. A: Space Group Symmetry, 4th ed. Dordrecht, Netherlands: Kluwer, p. 752, 1995.

Veysseyre, R. and Veysseyre H. "Point Groups of Five-Dimensional Space." Acta Cryst. A 58, 429-433, 2002.

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Weisstein, Eric W. "Point Groups." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/PointGroups.html