Corner flow, driven by differential heating at the vertical walls in a rectangular cavity, is studied. The flow is initiated from a strong vertical stratification of a Boussinesq fluid. The system Rayleigh number is large and the Prandtl number is O(1). Previous studies point to a considerable thickening of the boundary layer depth, which is referred to as the comer jet when the fluid moves vertically in and horizontally out near the comer. Some authors argue that the phenomenon is caused by an internal hydraulic jump. In some other investigations, the comer jet structure is shown to arise due to the temperature undershoots in the vertical boundary layer, which stem from the stable thermal stratification in the core. In this paper, the above controversy is resolved in part and a theoretical model for the comer flow is given. Furthermore, efforts are made to establish the condition for a smooth turning flow, without abrupt changes of the flow depth near the corner.