This study investigates the underlying mechanisms that initiate secondary flow in developing turbulent flow along a corner. This is done by theoretical examination of the total shear stress, which is the time-averaged product of instantaneous streamwise velocity U and the velocity Vn normal to the interface. The study shows that lines of zero total shear stress exist in the flow region, which delineate the region of secondary flow. Therefore, the flow region is dividable and eight vortices occur in a duct flow. The theoretical and experimental results show that the division line, separating the neighboring secondary currents in a corner, is not always identical to the bisector of the corner, but deviates from the corner bisector if the aspect ratio is b/h = 1. By simplifying Reynolds equation in the near-bed region, we find that theoretically a lateral variation of streamwise velocity initiates the wall-tangent flow that drives the vortex in the region bounded by zero total shear stress. A simplified method for estimating the vortex center, near-bed secondary velocity, and shape of secondary currents has been proposed, and a good agreement between the measured and predicted features is achieved.