  A duality relationship between certain brane configurations in type IIA and
type IIB string theory is explored by exploiting the geometrical origins of
each theory in M-theory. The configurations are dual ways of realising the
non-perturbative dynamics of a four dimensional N=2 supersymmetric SU(2) gauge
theory with four or fewer favours of fermions in the fundamental, and the
spectral curve which organizes these dynamics plays a prominent role in each
case. This is an illustration of how non-trivial F-theory backgrounds follow
from M-theory ones, hopefully demystifying somewhat the origins of the former.
