  To do realistic model building in type IIB supergravity, it is important to
understand how to fix D7-brane positions by the choice of fluxes. More
generally, F-theory model building requires the understanding of how fluxes
determine the singularity structure (and hence gauge group and matter content)
of the compactification. We analyse this problem in the simple setting of
M-theory on K3xK3. Given a certain flux which is consistent with the F-theory
limit, we can explicitly derive the positions at which D7 branes or stacks of
D7 branes are stabilised. The analysis is based on a parameterization of the
moduli space of type IIB string theory on T^2/Z_2 (including D7-brane
positions) in terms of the periods of integral cycles of M-theory on K3. This
allows us, in particular, to select a specific desired gauge group by the
choice of flux numbers.
