  A new approach to derivation of various effective-field approximation for
lattice spin models is presented. It is shown that it can give a number of
methods, including the DMRG method, that can be used to find generally
inhomogeneous solutions of 2D classical lattice problems. A method, closely
related the DMRG method but without necessity to perform any renormalization,
is derived, yielding results practically not different from the DMRG ones. The
matrix-product wave function of Rommer and \"Ostlund can be constructed from
the output of the method. The computational costs of all the derived methods
are smaller than those of the DMRG. Most of the results are applicable to the
1D quantum systems, as well.
