Physics: 230 space groups (Fedorov 1890, Schoenflies 1891) combine point group symmetry with lattice translations. Screw axis 2₁: 180° rotation followed by half-unit-cell translation — generates systematic absences (h00 reflections with h odd absent for 2₁ along a). Glide plane: reflection + half translation. Protein crystals cannot have inversion or improper rotations → 65 Sohncke groups. Most common protein space group: P2₁2₁2₁ (space group #19). X-ray crystallography uses systematic absences to determine space group from diffraction pattern before solving the structure.