In the case of an initially conical jet, we study the relation between jet collimation by the external pressure and large-scale morphology. We first consider the important length-scales in the problem, and then carry out axisymmetric hydrodynamic simulations that include, for certain parameters, all these length-scales. We find three important scales related to the collimation region: (i) where the sideways ram pressure equals the external pressure, (ii) where the jet density equals the ambient density and (iii) where the forward ram pressure falls below the ambient pressure. These scales are set by the external Mach number and opening angle of the jet. We demonstrate that the relative magnitudes of these scales determine the collimation, Mach number, density and morphology of the large-scale jet. Based on the analysis of the shock structure, we reproduce successfully the morphology of Fanaroff–Riley (FR) class I and II radio sources. Within the framework of the model, an FR I radio source must have a large intrinsic opening angle. Entrainment of ambient gas might also be important. We also show that all FR I sources with radio lobes or similar features must have had an earlier FR II phase.