  The aim of this work is to give a pedagogical derivation of polarization
observables for the annihilation reaction $e^+ + e^- \to N+ \bar N$. The
reaction mechanism is one photon + two photon exchange, the last is described
by axial parametrization. After deriving the general expressions for the cross
section of a binary process, the matrix element is written in terms of three
complex amplitudes. The method to derive polarization observables is detailed
and all expressions are given in terms of generalized form factors. The
strategy for determining physical form factors in annihilation reactions in
presence of two photon exchange is suggested, on the basis of model independent
properties of the relevant observables.
