It is a widely unknown or unrecognized fact that the vapor-liquid critical points of pure compounds and also of mixtures form a sail-like surface in the three-dimensional space of temperature, pressure, and molar volume or density. The available experimental critical points of all pure compounds lie on or near that critical surface with some exceptions caused mainly by extraordinary intermolecular interactions (e.g., strong acids or polar compounds like water). For this surface and therefore the relation among the three critical properties, simple relation equations, transformable and explicit in each property, were evaluated and optimized. With these equations the critical volume, pressure, or temperature can be calculated from the two remaining properties with an average absolute deviation of less than 5% over a set of recommended critical points for 421 organic compounds. This relation is also applicable for the critical properties of binary and multicomponent mixtures. Therefore, a unique tool is available for a consistency test of experimental or predicted data and for simple and reliable calculations of the often missing property, for example, the critical molar volume.