  We construct N=1 supersymmetric fractional branes on the Z_6' orientifold.
Intersecting stacks of such branes are needed to build a supersymmetric
standard model. If a,b are the stacks that generate the SU(3)_c and SU(2)_L
gauge particles, then, in order to obtain just the chiral spectrum of the
(supersymmetric) standard model (with non-zero Yukawa couplings to the
  Higgs multiplets), it is necessary that the number ofintersections a \circ b
of the stacks a and b, and the number of intersections a \circ b' of a with the
orientifold image b' of b satisfy (a \circ b,a \circ b')=\pm(2,1) or \pm(1,2).
It is also necessary that there is no matter in symmetric representations of
the gauge group, and not too much matter in antisymmetric representations, on
either stack. We provide a number of examples having these properties.
Different lattices give different solutions and different physics.
