In 1882 Thomas Carnelley observed that high molecular symmetry is associated with high melting point. The application of the rule to a number of different molecular crystals is discussed. The rule applies to different categories of crystal for different reasons, which can be explained by thermodynamic analysis. If the crystal is ordered, high melting point is usually due to high enthalpy change of fusion. If the crystal is disordered, high melting point is due to low entropy change of fusion. These effects can be displayed clearly on an enthalpy-entropy diagram.