  We consider the general free field theory such that system of equations of
motion includes a subsystem with a special property. If the subsystem is
considered by itself, it would be a topological field theory having no local
degrees of freedom. Various well-known field theories admit such a subsystem,
including Einstein's gravity. As the subsystem is a topological theory, the
general solution is a pure gauge, modulo global degrees of freedom. Gauge
symmetry of the subsystem of linear theory can be explicitly found, providing
the general solution to the subsystem. Substituting this solution into entire
system, we arrive at an equivalent field theory where the role of the field
variables is played by the gauge parameters of the topological subsystem. These
fields can be viewed as the potentials for the original ones. A general
procedure is proposed for constructing a "parent action" which includes the
fields of both dual formulations. At the level of this action, one can switch
between dual formulations by imposing appropriate gauge fixing conditions. This
general dualisation procedure is exemplified by massless and massive spin two
fields. For the massless case, the hook type tensor serves as a potential for
the metric, while for the massive case it is the fourth rank tensor with window
Young diagram.
