The boundary-layer calculation method of Bradshaw, Ferriss, and Atwell has been adapted to deal with the interaction between two shear layers with a change of sign of shear stress. Good agreement with experiments in symmetrical duct flow is found, using the same empirical input as in a boundary layer and assuming that the turbulence fields on either side of the duct can be superposed. The restriction to symmetrical flow is temporary and is a numerical rather than a physical simplification. In free jet flows, which have higher turbulence levels than ducts, small changes in empirical input are required to treat the interaction.