In some situations, a corrugated board package can experience loading of such a kind so that the corrugated core, or as it is most often referred to, the fluting, is loaded in transverse shear. This might cause the fluting to collapse and through this, cause a decrease in the bending stiffness. In this paper, an exact solution for the elastic instability of an infinite linear elastic strip with an initial curvature and loaded in pure in-plane shear is given. The solution is a modification of an existing solution for an isotropic strip. To experimentally investigate the accuracy of the model, a special device has been developed in which paper strips are loaded in pure in-plane shear to observe the buckling behavior. According to the experiments performed, the model seems to quantitatively well capture the main buckling behavior and lends some confidence to the theory. With use of the model it is easy to judge which deformation process most likely will occur, shear buckling or plastic deformation, and thereby allows one to structurally optimize the geometric ratio, i.e. the ratio between thickness and height of the fluting, when designing the corrugated core structure. Finally, it is illustrated that going below a certain critical thickness of the fluting may cause the structural strength of the board panel to decrease drastically.