The peculiarity called “dissimilation graphique” is recurrent in Egyptian writing systems, particularly during Old Kingdom (Dyn. III-VI, ca. 2700-2150 BC). At this time, plurality (grammatical and lexical) is indicated by the threefold of semograms, signs that have a semantic value and allow to understand the lexemes. Thus, if ideograms and classifiers are ordinary reproduced three times identically, “dissimilation” refers to occurrences where these three signs are different. For example, if bAk.w, “servants”, is usually followed by three men, in case of dissimilation, one of them can be replaced by a woman. In the same way, if mHy.t, “fish”, habitually takes three identical fish, a dissimilated version will show three different species. This Ph.D Thesis aims to analyse in a systematic way the process of “dissimilation graphique”, mostly found in hieroglyphic inscriptions of royal and private tombs, but also in some hieratic documents. Through this study, the supposed reasons of dissimilation are exposed. Consequently, Egyptian classification is newly enlightened. Moreover, in examining a great number of Old Kingdom texts, new analysis of the signs functions are proposed to clarify some unclear elements of egyptological Linguistics.