When the axial plane is a curviplanar surface in the fold profile or when hinge and axial plane are both folded similarly, the angle between the two relatively straight segments is called the axial plane angle g and the tan of this is a measure of nonplane noncylindricity of the fold. The term is after Williams and Chapman who gave a triangular diagram for the classification of such folds. The triangular diagram is reproduced here to again explain the axial plane angle.