Numerical modeling of fold development in a layered anisotropic medium under various geometries of rotational strain shows that the only necessary requirement for fold growth from an initial perturbation (fold instability) is a component of shortening in the plane of the layering. Three strain regimes, end-member forms expected in thrust belts or shear zones where strain departs from ideal progressive simple shear, were studied. Wrench shear gives rise to symmetrical folds with axes subparallel to the maximum extension direction. Combined wrench shear and thrust shear produces asymmetric folds with axes close to the extension direction. Strongly asymmetric folds may result from thrust shear together with a small component of shortening in the shear direction. In wrench shear, the folds develop without á component of extension in the profile plane.