The objective of Structural Design is to provide the structure that will permit the aircraft, whether military or civil, to do its job most effectively, that is with the least total effort, spread over the whole life of the aircraft from initial design until the aircraft is thrown on the scrap heap. There is thus no all-embracing simple criterion by which the success of the structural design can be judged. It is not sufficient to believe that percentage structure weight is of itself an adequate measure of effective design, either of the complete vehicle or of the structure itself; a well-known example is the provision of increased aspect ratio at the expense of structure weight, which may give increased fuel economy in the cruise and reduction in the total aircraft weight. Nevertheless, percentage structure weight is a useful measure, provided its limitations are recognised.