  We consider type IIB theory compactified on a two-sphere in the presence of
mutually nonlocal 7-branes. The BPS states associated with the gauge vectors of
exceptional groups are seen to arise from open strings connecting the 7-branes,
and multi-pronged open strings capable of ending on more than two 7-branes.
These multi-pronged strings are built from open string junctions that arise
naturally when strings cross 7-branes. The different string configurations can
be multiplied as traditional open strings, and are shown to generate the
structure of exceptional groups.
