  We present an effective field theory method to determine secondary massive
quark effects in jet production taking the thrust distribution for e+ e-
collisions in the dijet limit as a concrete example. The method is based on the
field theoretic treatment of collinear and soft mass modes which have to be
separated coherently from the collinear and ultrasoft modes related to massless
quarks and gluons. For thrust the structure of the conceptual setup is closely
related to the production of massive gauge bosons and involves four different
effective field theories to describe all possible kinematic situations. The
effective field theories merge into each other continuously and thus allow for
a continuous description from infinitely heavy to arbitrarily small masses
keeping the exact mass dependence of the most singular terms treated through
factorization. The mass mode field theory method we present here is in the
spirit of the variable fermion number scheme originally proposed by Aivazis,
Collins, Olness and Tung and can also be applied in hadron collisions.
