  We construct a generic mass model of a disk galaxy by combining the usual two
mass components, an exponential thin stellar disk and a dark halo for which we
alternatively assume one of the two most common mass distributions. We
construct its generic rotation curve, with two free parameters linked to its
shape and one to its amplitude, namely the halo characteristic scale lenght,
the fractional amount of Dark Matter and the total mass at one disk scale
lenght. We find that there is no need of a fine-tuning in the above parameters
to produce smooth, featureless rotation curves like the ones actually observed.
The rotation curves obtained from different mass distributions are
significanlty affected by the variation of the parameters, and show substantial
difference in their slope; however, they do not feature irregularities marking
the transition from a disk-dominated to a halo-dominated regime. On the other
hand we show, by direct analysis of a large number of plausible and different
mass models, that an observed rotation curve fulfilling well defined quality
requirements can be uniquely and properly decomposed in its dark and luminous
components; through a suitable mass modelling together with a
maximum-likelihood fitting method, in fact, the disentangling of the mass
components is accomplished with a very high resolution.
