Mountain systems, or orogens, result from collisional processes and are commonly curved in plan view. Two main models have been proposed explaining bends of orogens. Thomas (1977, 2006) explained curved mountain systems as reflecting the primary shape of the pre-collisional continental rifted margin. The alternative is that the bends result from deformation of previously more linear orogens, and hence reflect continental and perhaps lithosphere-scale strain. Identifying the processes that might result in the development of such large-scale structures, commonly referred to as oroclines is, however, difficult. Nonetheless, primary explanations of curved mountain belts, including the Cordillera and Appalachians of western and eastern North America, respectively, and the Variscan of Europe, fail to account for much paleomagnetic and paleo-stress data. Resolution of the debate is fundamental to understanding the paleogeographic and tectonic evolution of the Earth.