  Aims: We attempt to revise the period-amplitude (P-A) relationship of
Galactic Cepheids based on multi-colour photometric and radial velocity data.
Reliable P-A graphs for Galactic Cepheids constructed for the U, B, V, R_C, and
I_C photometric bands and pulsational radial velocity variations facilitate
investigations of previously poorly studied interrelations between observable
amplitudes. The effects of both binarity and metallicity on the observed
amplitude, and the dichotomy between short- and long-period Cepheids can both
be studied. Results: Large amplitude Cepheids with companions exhibit smaller
photometric amplitudes on average than solitary ones, as expected, while
s-Cepheids pulsate with an rbitrary (although small) amplitude. The ratio of
the observed radial velocity to blue photometric amplitudes, A_V_RAD/A_B, is
not as good an indicator of the ulsation mode as predicted theoretically. This
may be caused by an incorrect mode assignment to a number of small amplitude
Cepheids, which are not ecessarily first overtone pulsators. The dependence of
the pulsation amplitudes on wavelength is used to identify duplicity of
Cepheids. More than twenty stars previously classified as solitary Cepheids are
now suspected to have a companion. The ratio of photometric amplitudes observed
in various bands confirms the existence of a dichotomy among normal amplitude
Cepheids. The limiting period separating short- and long-period Cepheids is
10.47 days. Conclusions: Interdependences of pulsational amplitudes, the period
dependence of the amplitude parameters, and the dichotomy have to be taken into
account as constraints in modelling the structure and pulsation of Cepheids.
Studies of the P-L relationship must comply with the break at 10.47 instead of
the currently used `convenient' value of 10 days.
