How to read (and understand) Volume A of International Tables for Crystallography: an introduction for nonspecialists

Dauter, Z.; Jaskolski, M.

doi:10.1107/S0021889810026956

Figure 4
The molecule of methane, CH₄, with its H atoms labeled to guide the eye, can be transformed by the action of a proper twofold axis (upper panel) and by the action of the $[\overline 4]$ rotoinversion axis (lower panel). In addition, the molecule has (not shown) four threefold axes along the C-H bonds, and six mirror planes defined by the six possible H-C-H planes (H1-C-H2, H3-C-H4, H1-C-H3, H2-C-H4 etc.).