The 17 plane groups are classified as oblique, rectangular, square, and hexagonal. To describe the symmetry of 2D crystals of biological molecules, the plane groups are expanded to their equivalent noncentrosymmetric space groups. The 2D crystal plane corresponds to the 'ab' plane of the space group. . Enumerated space group descriptions include the plane group number in parentheses, the H-M plane group symbol, and the plane group class.