  We present a unified geometric framework for describing both the Lagrangian
and Hamiltonian formalisms of regular and non-regular time-dependent mechanical
systems, which is based on the approach of Skinner and Rusk (1983). The
dynamical equations of motion and their compatibility and consistency are
carefully studied, making clear that all the characteristics of the Lagrangian
and the Hamiltonian formalisms are recovered in this formulation. As an
example, it is studied a semidiscretization of the nonlinear wave equation
proving the applicability of the proposed formalism.
