The form of a single folded layer depends upon the geometry of each surface of a layer and in particular the curvature of each surface in the hinge area as well as away from It. The curvature of both the surfaces may be the same or different.

Ramsay (1967) has divided the folds into three classes depending on the relations of curvatures of surfaces bounding a layer and these are designated as Class 1, Class 2 and Class 3.

In class 1 folds the curvature of the outer are is less than that of the inner arc or the radius of curvature of the outer arc is greater than that of the inner arc. In class 2 folds the curvatures of both the arcs is the same, or the radii of curvature of both the arcs are of the same length. In class 3 folds the curvature of the outer are is greater than that of the inner arc or the radius of curvature of the outer are is less than that of the inner arc.

Class 1 folds were divided into subclasses A, B and C. The figure is after Ramsay and Huber 1983 with fold class boundaries (Yellow 1A, Red 1B, Blue 1C, hatched 2 and orange 3) demarcated on an experimentally produced single layer fold by Cobbold in paraffin wax.