A fourteenth century 'libro d'abaco' (codex L.VI.47 in Biblioteca Comunale degli Intronati di Siena, ca. 1325) is completely transcribed. The critical inquiry, both linguistic and mathematical, shows that it is the earliest mathematical manuscript in Pisan dialect, but no relevant connection with the Liber Abaci (written by Fibonacci about a century before) emerged. Nonetheless it is an interesting document of the vernacular teaching of mathematics, perhaps the most ancient written in Tuscany, and the first that was entirely analized as a linguistical relevant source. The article is actually divided in two èarts (1. introduction and text; 2. linguistic inquiry and glossary).
