In the paper we analyze the mutual influence of constructive and technological dimensional links on conditions of formation of the machining accuracy sizes. It is shown, that the formation the sizes from technological locating datum surface demands higher accuracy of the technological sizes, but in this case, the machining allowances are more stable. At the formation the sizes by the means of transitions from technological locating datum surface to adjustment surface, the accuracy sizes is achieved without necessity of increase of an accuracy of the technological sizes, but thus, it is observed a growth of tolerances of the machining allowances and decreasing of the accuracy if some surface will not be machined. The dimensional optimality is not characterized only by the number of technological sizes, but it is necessary to take into account the growth (sometimes complicated) of the technological sizes accuracy (a case of formation of the sizes as closing link of dimensional chains).