A fold surface seen perpendicular to its shortening direction can be divided into "hinge" and "limb" portions, the limbs are the flanks of the fold and the hinge zone is where the limbs converge. Within the hinge zone lies the hinge point, which is the point of minimum radius of curvature (maximum curvature) of the fold. The crest of the fold represents the highest point of the fold surface whereas the trough is the lowest point. The inflection point of a fold is the point on a limb at which the reverses; on regular folds, this is the midpoint of the limb.