The rotation operations (both proper and improper) occur with respect to line called an axis of rotation. A proper rotation is performed by rotating the molecule 360°/n, where n is the order of the axis. If the resulting configuation is indistinguishable from the original, we say there exists an n-fold proper rotation axis (or Cn axis) in the molecule.
Each of the following molecules contains one or more proper axes:
Proper rotation axes: