Relativistic properties of a Lagrangian density are compared with those of a Hamiltonian density. It is proved that a Lagrangian density and a Hamiltonian density undergo different Lorentz transformations. This outcome is a theoretical element that has been unnoticed for a very long time. It is also proved that this theoretical element plays a crucial role in the structure of weak interactions theory. In particular, it is shown that the theory that uses this element is overwhelmingly superior over the Standard Model electroweak theory.