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This is the thickness of a layer between the two tangents of the same limb dip value measured parallel to the axial plane trace of the fold. The procedure of measurement is the same as for the This difers from orthogonal thickness the difference being that the thickness measured is not the perpendicular distance between the two tangents, but the distance parallel to the axial plane trace.

While studying the geometry of a folded layer (or layers), the thickness measured parallel to the axial plane trace of the fold in its profile between the two tangents drawn for any chosen limb dip for the two arcs of the fold is called axial plane thickness designated by Ta, in contrast to the orthogonal thickness measured as the least distance between the two tangents and designated ta.


T'>sec a Subclass 1A folds

T'=sec a Subclass 1B folds

T'>1, T'<sec a Subclass 1C folds

T'=1 Class 2 folds

T'<1 Class 3 folds