The boundary layer of an incompressible fluid in the domain ahead of the departure of the free streamline from the surface of a smooth body or a break-point of its generator, is considered. The potential of the external irrotational velocity field is taken from the theory of jet flows. It is assumed with respect to the initial value of the surface friction that its order can vary over a wide range, while remaining finite, or taking extremely large values. The boundary layer in the preseparation domain always admits of a unified mathematical treatment, in which the initial surface friction plays the role of a parameter.