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 C_{n} axis) in the molecule.

Each of the following molecules contains one or more proper axes:

Proper rotation axes: