  We study junctions between confining strings. We show that the effective
theory of such junctions is very predictive with only one new parameter, the
junction's mass, controlling the first couple of terms in the expansion in the
system size. By open-closed duality these considerations about the baryon
junction map to interaction vertices of closed strings. Therefore, we calculate
the interaction vertices of closed strings in theories such as Yang-Mills
theory. We find some surprising selection rules for string interactions in 3+1
dimensions. Requiring perturbative stability and that the string coupling is
weak, we suggest constraints on the junction's mass.
