This paper deals with the formation and breakdown of liquid jets from different nozzle designs with square cross section. The jet formation is quantitatively examined in terms of a scaled viscosity and pressure transient for a convergent nozzle design. The study also shows, that a divergent nozzle outlet should be avoided, because it may cause asymmetric disturbances on the jet. Two different mechanisms of jet breakdown occur in dependence on the nozzle geometry. Curves of the critical Weber number vs. Reynolds number are calculated.