  The large mass limit of QCD uncovers symmetries that are not present in the
QCD lagrangian. These symmetries have been applied to physical (finite mass)
systems, such as B and D mesons.
  We explore the validity of this approximation in the 't Hooft model
(two-dimensional QCD in the large-N approximation). We find that the large mass
approximation is good, even at the charm mass, for form factors, but it breaks
down for the pseudoscalar decay constant.
