  String theory gives S matrix elements om which is not possible to read any
gauge information. Using factorization we go off shell in the simplest and most
naive way and we read which are the vertices suggested by string. To compare
with the associated Effective Field Theory it is natural to use color ordered
vertices. The alpha'=0 color ordered vertices suggested by string theory are
more efficient than the usual ones since the three gluon color ordered vertex
has three terms instead of six and the four gluon one has one term instead of
three. They are written in the so called Gervais-Neveu gauge. The full
Effective Field Theory is in a generalization of the Gervais-Neveu gauge with
alpha') corrections. Moreover a field redefinition is required to be mapped to
the field used by string theory. We also give an intuitive way of understanding
why string choose this gauge in terms of the minimal number of couplings
necessary to reproduce the non abelian amplitudes starting from color ordered
ones.
