  In the present paper we continue the project of systematic classification and
construction of invariant differential operators for non-compact semisimple Lie
groups. This time we make the stress on one of the main building blocks, namely
the Verma modules and the corresponding parabolic subalgebras. In particular,
we start the study of the relation between the parabolic subalgebras of real
semisimple Lie algebras and of their complexification. Two cases are given in
more detail: the conformal algebra of 4D Minkowski space-time and the minimal
parabolics of classical real semisimple Lie algebras.
