In this paper, the Raoult law (C R. Acad. Sci. Ser. C 1887, 104, 1430), the Zdanovskii-Stokes-Robinson (ZSR) rule (Trudy Solyanoi Laboratorii Akad. Nauk SSSR 1936, No. 6, 5; J. Phys. Chem. 1966, 70, 2126), and two extended ZSR rules of Wang (Acta MetalL Sinica 1980, 16, 195; Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1045) are presented in a unified way. Similar to the Raoult law, which was first noted empirically in pyridine solutions, the ZSR rule for isopiestic mixed electrolyte and nonelectrolyte aqueous solutions can be extended to every kind of liquid and solid solutions such as organic mixtures, aqueous and nonaqueous electrolyte and nonelectrolyte solutions, liquid and solid alloys, molten salt mixtures, slags, and nonstoichiometric solid solutions, resulting in two extended ZSR rules of Wang. Although the solutions obeying the Raoult law (or the related classically ideal solution model) are very few as compared with the classically nonideal solutions, one classically ideal solution {B+C+...+Z} may mix with many pure components A to form the classically nonideal solutions {A+B+C+...+Z} following the first extended ZSR rule (or the related partial ideal solution model) or mix with much more nonideal solutions {A1+A2+...+Aq} to form the classically nonideal solutions {A1+A2+...+Aq+B+C+...+Z} obeying the second extended ZSR rule (or the related ideal-like solution model). This would indicate that the ZSR rule and its extended forms are as important as the Raoult law for multicomponent systems.