  We interpret instantons on a torus with twisted boundary conditions, in terms
of bound states of branes. The interplay between the SU(N) and U(1) parts of
the U(N) theory of N 4-branes allows the construction of a variety of bound
states. The SU(N) and U(1) parts can contribute fractional amounts to the total
instanton number which is integral. The geometry of non-self intersecting
two-cycles in $T^4$ sheds some light on a number of properties of these
solutions.
