- Centre of symmetry (i): The center of symmetry i is a point in space such that if a line is drawn from any part (atom) of the molecule to that point and extended an equal distance beyond it, an analogous part (atom) will be encountered. Thus, the molecule 3,6-dimethylpiperazine-2,5-dione has centre of symmetry (sometimes referred to as centre of inversion) running through the centre of the molecule.
- Plane of symmetry (σ): A plane of symmetry is a reflection plane which brings into coincidence one point of the molecule with another one through the mirror reflection. Thus, glyoxal has a plane of symmetry running through the molecular plane.
- Axis of symmetry (Cn): Symmetry axis Cn, also called n-fold axis, is an axis which rotates the object (molecule) around by 360°/n, such that the new position of an object is superimposable with the original one. For example, (1Z,4E,8E)-3,7,11-trimethylcyclo-dodeca-1,4,8-triene has 3-fold rotation axis.
