One of the ways by which the geometry of can be studied in fold profile in by drawing dip isogons. These are lines of equal limb dip on adjacent surfaces. Between two adjacent surfaces, the dip isogons drawn are rectilinear or straight line segments. The procedure of constructing dip isogons is fairly simple. Tangents of various values of limb dip , alppha , are constructed on both outer and inner arcs, and for all surfaces in case of a multilayered folded complex usually at some fixed interval and the points where the tangents of the same limb dip value touch the folded surfaces joined. In some multilayered fold profiles. some of the dip isogons for adjacent limbs may even be joined. This is particularly possible for folds in which the curvatures of different layers markedly vary.
In class 1 folds dip isogons are convergent towards the axial plane trace of the fold or towards the core of the fold in passing from outer to inner arc. Most of the class 1 folds are concentric or parallel folds. In class 2 folds, since curvatures of both the arcs is the same, dip isogons are parallel to the axial plane trace of the fold; in class 3 folds the dip isogons diverge away from the axial trace as they traverse from outer to the inner arc.
Class 1 folds can be subdivided into three subclasses called sub-class 1A, sub-class 1B and sub-class 1C. In sub-class 1A folds the dip isogons are very strongly convergent and dip isogon for any value of alpha makes an angle beta with the axial plane trace which is always greater than the alpha value of that dip isogon. For example a dip isogon of 40° value makes an angle b of 48° with the axial plane trace which is greater than a value of 40°. In subclass 1B folds, the thickness of the layer remains uniform and dip isogons are perpendicular to layer surfaces. Dip isogons for any value of alpha make an angle with the axial plane trace, beta which to equal to alpha. In subclass 1C folds the dip isogons are weakly convergent and any dip isogon such as one for 20° a value, makes an angle beta with the axial place trace <20 for this isogon. In class 2 folds, since isogons are parallel to the axial plane trace, beta value in simply non-existent or it has a value of zero or 180°. In class 3 folds the dip isogons are divergent to the axial plane trace and take negative values which could be greater or less than alpha depending upon the difference in the degree of curvature of the two arcs.If adjacent isogons are inclined towards each other, curvature of the outer arc between the isogons is less than that for the same region on the inner arc. If adjacent isogons are parallel the curvature of the outer are between the isogons is the same as that of the inner arc. If adjacent isogons are inclined away from each other, the curvature of the outer arc between the isogons exceeds that of the inner arc. This implies that in a single fold all isogons may not be convergent or all divergent but that some may converge & others diverge because of the complexities involved in the generation of fold structures.