  We study curves of marginal stability (CMS) in five-dimensional N=1 E_N
theories compactified on a circle using the D3-brane probe realization. In this
realization, BPS states correspond to string webs in the affine E_N 7-brane
background and junction positions of the webs deteremine CMS. We find that
there exist string webs involving infinitely many junctions. Consequently the
E_N theories have infinitely many CMS. We also find that there exists a
transition from open strings to string webs involving loops. The transition
describes a new phenomenon occurring on CMS.
