Hinge, hinge line, limb, axial plane, interlimb angle, cylindrical fold কী?

On a folded surface, the hinge is the zone of greatest curvature and the hinge line the line that joins its points of greatest curvature; the limbs are the less curved parts on either side. The axial surface of a fold is the surface that joins the hinge lines of its successive folded surfaces; when it is a plane, it is the axial plane. The interlimb angle $\iota$ is the angle between the two limbs, measured in the profile, the plane perpendicular to the hinge line, between the tangents to the limbs at their points of inflection; $0 \le \iota \le 180{}^{\circ}$. A fold is a cylindrical fold when its surfaces are generated by a straight line moved parallel to itself; the direction of that line is the fold axis of Definition 4.14, and every hinge line of a cylindrical fold is a straight line parallel to it.