  We construct the spin formalism in order to deal in a direct and natural way
with processes involving transversity which are now of increasing popularity.
The helicity formalism which is more appropriate for collision processes of
definite helicity has been so far used also to manage processes with
transversity, but at the price of computing numerous helicity amplitudes
generally involving unnecessary kinematical variables.In a second step we work
out the correspondence between both formalisms and retrieve in another way all
results of the helicity formalism but in simpler forms.We then compute certain
processes for comparison.A special process:the quark dipole magnetic moment is
shown to be exclusively treated within the spin formalism as it is directly
related to the transverse spin of the quark inside the baryon.
