At first glance, to the geometrically unskilled eye, the geometric construction may cause the sense not to be understandable but after a while also an unexperienced should understand and assimilate the structure and how to apply it. This also might have been the case for the early violin makers. Learning to adopt the geometry to its utmost, making templates to check a number of cross section, is certainly not easy. Furthermore it is not easy without a good three dimensional imagination of the concept to design an outline. This could be the reason why it was permitted to a few to fully realize this "model" to its perfection. It almost may take a life time by skill to learn and understand that it is necessary to follow the "model" in order to make good instruments. Those who only copied without the necessary understanding of the underlaying concept, distorted the necessary shape of the shell, and did not achieve the same physical results. They certainly did not fail in handicraft but understanding and knowledge of the needed geometric coincidence. Only the shape of the "base line" and its location in relationship to the longitudal axis have to be remembered. This might very well be the reason why no drawing has ever been found. The secret is gone with the mind of the master when he died. The complexity of the shape of the instrument almost automatically asks for a complex answer. Therefore scientists concentrate and study small elements of the instrument in order to come to a final underlying concept by trying to put all pieces together. The geometric model provides a comprehensive discipline in the design of the instrument and connects it all together. No dimensions can be changed without disarranging the whole concept. The congruence not only of the plane (2D) geometry in addition to the congruence of solid (3D) geometry is remarkable. No single shape or size can be changed disrupting the whole concept. However, it leaves the violin maker a free hand regarding the size of the instrument, the size and the shape of the scoop and outline. I do not deem it possible to achieve this concept by mere intuition; the geometric model is required in order to accomplish such a complex shape. Even in this way it is no easy task to obtain the desired properties with precision. In my experience the dual "element" and mirror-image property of the isolines cannot be verified by visual skills; isoline deviation by about 0.5--1.0 mm (X and Y direction) at a given point may cause a deviation of less than 0.1--0.3 mm in hieght (X) of the arch. It seems obvious to me that the acoustical quality is largely dominated by the mechanical function of the shell and the various possible arrangement of scoop and outline. This may explain why instruments by Stradivarius and Guarneri del Gesu generally share the same arch properties as required by the geometric model, while differing as to outline and scoop. Small dissimilarities between the geometric model and the finished instrument will always crop up in the final stage of instrument making.The thickness of the shell at a given point or area has to be determined taking the properties of the wood and physical properties of the shallow shell into account. Either the built-in physical properties of the shallow shell as well as the properties of the wood are known. The combination makes it even more difficult to accomplish a high quality result. Thus far only practical experience, in combination with help of a mechanical-accoustical equipment revealing axact frequency-modes of free plates helps the violin maker to determine where to shave away material in order to achieve a specific frequency-mode. This "Ghladni method" developed by Carleen Hutchins to be usable for the violin maker is thus far the most accurate method to come forward in the process of determing thicknesses of plates in the "tuning process" on free plates, deciding the stiffness in different bending directions as translated to "frequency-modes". Unfortunately there is no accordance in function when the plates are glued to the rib. The function at that stage is still shrouded in mystery since the number of variables of shell shape and thereby different properties cannot be taken in account to give clear answers on what exactely would be changed to get optimal physical and euphonious results. Is it possible that the leak of knowledge of this geometric model with it's very special shaped arch is part of the lost secret and the reason that we more than 250 years after Stradivarius's death still can't make an instrument with the same quality as those of the early masters.