About the book: 2021 / Einstein laid two principles as the basis for SRT. However, in order to obtain the Lorentz transformations and all the relativistic consequences from them, only one principle (postulate) is sufficient - the invariance of the speed of light. This principle is the root cause of the Lorentz transformations, the only, necessary and sufficient condition for their derivation, as well as for the proclamation of the principle of relativity and equality of all inertial frames of reference. Obtaining Lorentz transformations from the principle of relativity is possible, but it is necessary to take into account the principle of constancy of the speed of light. The reason for SRT is the invariance of the speed of light. Einstein put two principles in the basis of SRT. However, in order to obtain the Lorentz transformations and all the relativistic consequences from them, only one principle (postulate) is enough — the invariance of the speed of light.