  We present a definite formulation of the Principle of General Covariance
(GCP) as a Principle of General Relativity with physical content and thus
susceptible of verification or contradiction. To that end it is useful to
introduce a kind of coordinates, that we call quasi-Minkowskian coordinates
(QMC), as an empirical extension of the Minkowskian coordinates employed by the
inertial observers in flat space-time to general observers in the curved
situations in presence of gravitation. The QMC are operationally defined by
some of the operational protocols through which the inertial observers
determine their Minkowskian coordinates and may be mathematically characterized
in a neighbourhood of the world-line of the corresponding observer. It is taken
care of the fact that the set of all the operational protocols which are
equivalent to measure a quantity in flat space-time split into inequivalent
subsets of operational prescriptions under the presence of a gravitational
field or when the observer is not inertial. We deal with the Hole Argument by
resorting to de idea of the QMC and show how it is the metric field that
supplies the physical meaning of coordinates and individuates point-events in
regions of space-time where no other fields exist. Because of that the GCP has
also value as a guiding principle supporting Einstein's appreciation of its
heuristic worth in his reply to Kretschmann in 1918.
