Numerical and experimental investigations about tandem blades have often been performed for the 2D flow of tandem cascades. The open literature suggests that tandem blades may be superior to single blades, especially for large turning angles. However, the investigations up to now never involved the losses, arising at the sidewall. Less information about the flow structure near the sidewall are available. This raises the question whether the geometry that generates minimal losses for the 2D flow also generates minimal losses near the sidewall for large turning angles. This paper deals with the 3D flow in the region of the sidewall of a high turning tandem cascade with low aspect ratio. For the numerical and experimental investigation two tandem cascades were designed using empirical correlations of Lieblein and Lei. Starting with these tandem cascades the configuration of the individual profiles is determined that generates minimal loss at the wall. Based on the flow topology the loss generation is identified and explained. The images of the flow topology are created on the basis of numerical and experimental oil pictures. The procedure and results are discussed in detail. In particular the occurrence of flow phenomena like corner stall of the 3D boundary layer is discussed. Finally design rules for the spacing ratio of the forward and afterward blades as well as the percent pitch of the blades are given. The examinations are performed with tandem cascades turning the flow from approximately 50° at a Reynolds number of 8 · 105. In every cascade the load split is constant (50%). The profiles are NACA65 with circular camber lines.