From Online Dictionary of Crystallography
A symmetry element is a lattice element about which a symmetry operation is performed. Symmetry elements are classified on the basis of the dimensionality N of the space on which they act, the upper limit on the dimensionality of the symmetry element being N-1.
The only symmetry element that exists in this space is the reflection point (mirror point).
In this space, two types of symmetry elements exist: zero and one-dimensional:
- rotations points
- reflection lines (mirror lines)
The inversion centre does not exist in spaces of even number of dimensions.
In this space, three types of symmetry elements exist: zero, one- and two-dimensional:
- inversion centres
- rotations axes
- reflection planes (mirror planes)