def bending_stress(My, Iy, z, Mz=None, Iz=None, y=None):
    """Bending stress in principle system

    :param float My: Moment in the y direction
    :param float Mz: Moment in the y direction
    :param float Iy: Area moment of inertia for the y direction
    :param float Iz: Area moment of inertia for the zeta direction
    :param float z: Coordinate
    :param float y: Coordinate

    :returns: Bending Stress in cross section
    :rtype: float
    """

    if Mz is None:
        return (My / Iy) * z
    else:
        return (My / Iy) * z, -(Mz / Iz) * y