Abstract: The Drucker`s postulate is the basis of the normality rules in plasticity, but a lot of experiments indicate that the rock and soil materials do not follow the Drucker`s postulate, and that the non-normality rules are fitter for the experiments. The problem is: what is the theoretical basis of the non-normality rules? This paper proves that the Drucker`s postulate is not suitable for the friction materials such as geological materials through the process that the stress cycling of a simple friction body does work, and that the normality rules in the stress space are also not suitable for the friction materials, hence the Drucker`s postulate is not the axiom that must be followed, and the non-normality rules can be used. It is also proved that the Ильюшина postulate that the strain is used as the state variable has a larger scope of application.