John Ramsay proposed a classification scheme for folds that is used to describe folds in profile based upon curvature of the inner and outer lines of a fold, and the behavior of dip isogons.
dip isogon: a line that connects points of equal inclination or dip on the outer and inner bounding surfaces of a folded layer
Class 1 - Dip isogons converge downward towards axial surface, signifying that the curvature of the outer arc is less than that of the inner arc
Class 1A - Limbs thicker than hinges
Class 1B - Layer thickness constant; parallel fold
Class 1C - Limbs thinner than hinges
Class 2 - Dip isogons are parallel, signifying that the curvature of the outer arc exactly matches the curvature of the inner arc; similar fold
Class 3 - Dip isogons diverge downward towards axial surface, signifying that the curvature of the outer arc is greater than that of the inner arc
Class 1B (parallel) and Class 2 (similar) folds are the most common folds seen in the field. Concentric folds are a special case of Class 1B (parallel) folds where the outer and inner bounding surfaces define arcs with a common center of curvature. These types of folds are common in upper crustal tectonic settings, where most deformation occurs by processes that only permit limited ductile flow of rock. Most of the deformation is accommodated by slip on bedding or layer boundaries (flexural slip folding). Class 2 (similar) folds have relative thinning of the limbs and thicking of the hinges. These types of folds are common in metamorphic terranes, where most deformation occurs by processes that permit extensive ductile flow of rock.
Sources:
Folding and Fracturing of Rocks, John G. Ramsay, 1967
Structural Geology of Rocks, 2nd Edition, George H. Davis & Stephen J. Reynolds, 1996
http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-113-structural-geology-fall-2005/lecture-notes/part6_dctl_fldfb.pdf
