Abstract: This study examines the working method of John Chortasmenos in the Vaticanus Urbinas graecus 80, ff. 134r-143v. Through a precise analysis of their scientific content – associable with Cleomedes, Euclid and Nicomachus of Gerasa – the study emphasises that these folios convey the result of a complex reading whose stages of completion have been sought in the manuscript traditions of each treatise. It shows that the sources from which John Chortasmenos works are both ancient and medieval. Moreover, the material presentation of these folios, particularly the diagrams and tables of numbers, indicates that they are didactic records intended for practical use in a way that differs from that of the ancient treatises used by Chortasmenos.