[eng] This paper focuses on root incompatibilities in Proto-Semitic and examines the importance of these laws with regard to historical root reconstruction. As is well known, these rules can only be applied to verbal roots, not to derivative forms and affixed forms. The importance of these structural incompatibilities consists, then, in the fact that they reduce the possible number of combinations of the triconsonantal bases. Excluding onomatopoeic roots and loan words, these laws of incompatibility are fully regular in the verbal roots (but not in the nominal ones) and, therefore, do not have exceptions, as in all phonological laws. The structure of the Semitic verbal roots is, then, absolutely conditioned by these restrictions of incompatibility. These rules are universal in character and apply also to the different families of the Afro-Asiatic and Indo-European languages. The restrictions of incompatibility are a tool of great importance in the historical reconstruction of the roots (especially, of the verbal roots in Semitic).