The textbook is a discourse genre whose communicative predominant function is that of regulating the insertion of learners into a specific area of knowledge. From this perspective, it is a genre of great relevance in the context of disciplinary literacy. Taking as context of observation the textbooks of four disciplinary areas: Psychology and Social Work, corresponding to the social and human sciences; and Chemistry and Construction Engineering, corresponding to the basic sciences and those of the engineering (Corpus PUCV-2006) and using as a toot of linguistic analysis the Sistemic Functional Grammar, specifically, the system of obligative modulation; this study pretends to describe and characterize the textbook genre, from the variation that the probabilistic profile of lexicogramatical configurations that, in Spanish, realize obligative modulation of the clause present (Gutierrez, 2007). For the exploitation of the corpus, we used the tagger El Grial (www.elgrial.cl) that is the one that administers the registers used in this study. The probabilistic estimation is developed through conditioned probabilities and the variation of the system, in the frame of the multiregister corpus is estimated on a global error of 3%. The principal results indicate that in the context of the textbook direct and congruent configurations are preferred to construct regulation, which reveals that the relation between expert and apprentice is distant. In turn, this constitutes a characteristic of the tenor of the genre, associated with the specific communicative situation in which this genre is used: teaching.