  Intersecting stacks of supersymmetric fractional branes on the Z_6'
orientifold may be used to construct the supersymmetric Standard Model. If a,b
are the stacks that generate the SU(3)_{colour} and SU(2)_L gauge particles,
then, in order to obtain {\em just} the chiral spectrum of the (supersymmetric)
Standard Model (with non-zero Yukawa couplings to the Higgs mutiplets), it is
necessary that the number of intersections a \cap b of the stacks a and b, and
the number of intersections a \cap b' of a with the orientifold image b' of b
satisfy (a \cap b,a \cap b')=(2,1) or (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. Fractional branes
having all of these properties may be constructed on the Z_6' orientifold. We
construct a (four-stack) model with two further stacks, each with just a single
brane, which has precisely the matter spectrum of the supersymmetric Standard
Model, including a single pair of Higgs doublets. However, the gauge group is
SU(3)_{\rm colour} x SU(2)_L x U(1)_Y x U(1)_H. Only the Higgs doublets are
charged with respect to U(1)_H.
