  The aim of this work is to study the quotients for the diagonal action of
SL_3(C) on the product of n-fold of \mathbb{P}^2(C): we are interested in
describing how the quotient changes when we vary the polarization (i.e. the
choice of an ample linearized line bundle). We illustrate the different
techniques for the construction of a quotient, in particular the numerical
criterion for semi-stability and the ``elementary transformations'' which are
resolutions of precisely described singularities (case n=6).
