Abstract: A method and apparatus for folding the major flaps of cartons uses a folding rail having entry, transitional and exit sections which include vertical, curvilinear and horizontal surfaces, respectively, such that the transitional and exit sections are machined from a solid block of metal to have a curvilinear surface of a particular curvature developed from three-dimensional modelling on a computer aided design system in order to consistently square the cartons in a folding operation. The folding rails are positioned in a flap closure station such that a leading folding rail is mounted on a stationary mounting block and a trailing folding rail is mounted on a translatable folding rail in order for the folding rails to be moved closer together or farther apart to fold cartons of different widths.