Heuristic is the first step to provide a good critical text edition. It allows to collect all the manuscripts of a work, as well as to order them according to their content. This preliminary work phase is also important in order to obtain essential information about the genre and type of a work, thus providing useful new starting points for a comprehensive study. The paper provides the first results of this work phase within the project that involves editing, translating and providing a commentary on Dionysius of Halicarnassus’ ‘Epistula ad Ammaeum II’. All the manuscripts of the ‘Epistula’ can be divided into two groups, which are almost always characterised by two specific and different compositions: the first one allows us to consider the ‘Epistula’ as a rhetorical treatise providing rhetorical and grammatical remarks about the style of Thucydides; the second clarifies the importance of the ‘Epistula’ as necessary support to the comprehension of the ‘Historiae’ of Thucydides. Within these two groups of manuscripts there are some witnesses which present an unusual composition, such as the Parisinus gr. 2755. This case study sheds light on interesting and sometimes neglected aspects of the ‘Epistula’ and provides a complete analysis of its genre and use.