  We show that in classical mechanics, as well as in nonrelativistic quantum
mechanics the equation of the relative motion for a two-body bound system at
rest can be replaced by individual dynamical equations of the same kind as the
first one, but with different parameters. We assume that in relativistic
quantum mechanics the individual equations are Dirac equations with modified
parameters in agreement with the individual Schr\"odinger equations. We find
that products of solutions to the individual equations with correlated
arguments are the quantum analogues of the classical representation of a bound
system and represent suitable models for the bound state wave functions. As
validity test for the new representation of bound states we suggest to use some
observable differences between this one and the representation in terms of
relative coordinates.
