  In this paper a set of canonical collective variables is defined for a
classical Klein-Gordon field and the problem of the definition of a set of
canonical relative variables is discussed. This last point is approached by
means of a harmonic analysis is momentum space. This analysis shows that the
relative variables can be defined if certain conditions are fulfilled by the
field configurations. These conditions are expressed by the vanishing of a set
of conserved quantities, referred to as supertranslations since as canonical
observables they generate a set of canonical transformations whose algebra is
the same as that which arises in the study of the asymptotic behaviour of the
metric of an isolated system in General Relativity.
