In recent years, greater attention has been paid to the relationship between Dante's epistolary practice and the theory and practice of ars dictaminis, particularly through the identification of parallels with texts contained in summae dictaminis like the so-called letters of Pier della Vigna. The present paper aims to show how a systematic survey of the syntagms present both in Dante's letters and in the most diffused collections of letters (papal, Sicilian, communal) of the thirteenth century can help to understand the way Dante worked. From a database of some 3500 texts, we can highlight more than a hundred strict parallels, but also show how Dante was able to choose syntagms by practising a combinatorial technique inspired by the microformularism already present in thirteenth-century letters. An analysis of this type not only allows us to highlight the proximity between Dante's letters and certain sources little used by research on Dante (papal letters of Clement IV, "pseudo-papal" letters of Riccardo da Pofi…). It also makes it possible to measure the distance between prestigious letters of Frederick II and their possible imitations by Dante, who in some cases may not have wanted to draw too slavishly on these famous examples.