The construction of models of crystal structures presented no very great difficulties as long as the investigated structures were relatively simple, commonly cubic ones. One method was to slide metal rods into a heavy base board and to fasten beads or balls on these rods at proper intervals. Later more attractive models were made with wooden balls held together by metal rods which at the same time represented the coordinate valency bonds. The difficulty in constructing such models lies in the drilling of holes in the wooden balls at proper angles and in the centering of the balls during the drilling. Besides, with the coming of more complicated structures this method became so difficult and expensive that it is practically prohibitive now.