  By exploting the data of three large surveys (WINGS, NFPS and SDSS), we
present here a comparative analysis of the Fundamental Plane (FP hereafter) of
the early-type galaxies (ETGs) belonging to 59 galaxy clusters in the redshift
range $0.04<z<0.07$.
  We show that the variances of the distributions of the FP coefficients
derived for the clusters in our sample are not consistent with the hypothesis
of universality of the FP relation. By investigating the origin of such
remarkable variances we find that, besides a couple of obvious factors, such as
the adopted fitting technique and the method used to measure the photometric
and kinematic variables, the coefficients of the FP are strongly influenced by
a number of things, mainly related to the distribution of photometric/kinematic
properties of galaxies in the particular sample under analysis. For instance,
the $a$ coefficient derived for the whole sample of ETGs, turns out to decrease
when faint galaxies are progressively removed from the sample, suggesting that
bright and faint ETGs have systematically different FPs, likely because of
different mechanisms of galaxy formation. In general, by comparing mock cluster
samples with the real one, we conclude that the observed variances of the FP
coefficients cannot be attributed just to statistical uncertainties. We
speculate that the FP is actually a bent surface, which is approximated by
different planes when different selection criteria, either chosen or induced by
observations, are acting to define galaxies samples.
  We also find ...
