Outer arcs of folds are also called extradoses and inner arcs the intradoses. The relative radii of curvature define the fold class. Extrados radius of curvature> intrados radius of curvature: Class 1 folds

Extrados radius of curvature=intrados radius of curvature: Class 2 folds

Extrados radius of curvature<intrados radius of curvature: Class 3 folds