Finite element method allows solving a large variety of engineering and mathematical physics problems. Historically, the first application of FEM was in mechanics, that affected its terminology and first interpretations. Indeed, the essence of the method can be described as follow: the continuous medium is replaced by the equivalent hinged system, that allows representing the system as a statically indeterminate. Solving statically indeterminate hinged rods system is a straightforward and well-known problem. This simplified treatment contributed to the widespread of the method, while strictly speaking this treatment is incorrect. Precise mathematical handling of the method was developed much later of first successful method applications, but it enabled extension of the method to a wide range of problems, not only mechanical.