  The construction of effective Lagrangians commonly involves the application
of the `classical equation of motion' to eliminate redundant structures and
thus generate the minimal number of independent terms. We investigate this
procedure in the framework of chiral perturbation theory. The use of the
'classical equation of motion' is interpreted in terms of field
transformations. Such an interpretation is crucial if one wants to bring a
given Lagrangian into a canonical form with a minimal number of terms. We
emphasize that the application of field transformations not only eliminates
structures, or, what is equivalent, expresses certain structures in terms of
already known different structures, but also leads to a modification of
coefficients of higher--order terms. This will become relevant, once one
considers effective interaction terms beyond next--to--leading order, i.e.,
beyond $O(p^4)$.
