This article is concerned with the study of medieval manuscripts and the determining of the features of protographs based on the graphic and orthographic systems of their scribal copies. The study involves three manuscripts created in Nuremberg in the 15th century. The texts of the mentioned manuscripts are copies from the first two complete translations of one of the medieval treatises. Based on the study of the graphic and orthographic systems of these texts, as well as some of their morphological features, the author of the article makes an attempt to establish the relationship between the texts under study, as well as to restore some of the features of their protographs. The sources of influence on the formation of the graphic and orthographic portrait of the scribe were identified as a result of the study of the graphic and orthographic features of the texts. Among them the author identifies the written tradition of the scriptorium, features of the local dialect and dialect areas, as well as characteristic features of the protograph. A separate group includes such a source of variability as the individual preferences of the scribe. Analysis of the graphic and orthographic portrait of each of the scribes who participated in the creation of the texts under study allows us to determine the approximate time and place of creation of the protograph, as well as changes characteristic of each stage of conscious processing of the text. As a result of the study, a diagram was drawn up that clearly demonstrates the relationships between the texts under study.