Accounting in the Middle Ages is mainly known through collections of account books from this period, and most of them have been largely commented on for nearly a century. Besides this body of literature, the rich medieval documents written in Latin also contain abundant analyses on the keeping of these records. The legalists of this time did not write any specific manual on the art of keeping these account books or about a law specific to them. Unfortunately, their countless recommendations are scattered and contained in very various and sometimes rare manuscripts. These sources are however significant enough to justify limiting them to those prior to 1494, date of the famous formalization of double entry accounting proposed by Luca Pacioli. We can try to identify a possible influence of the art of the double entry from this documentation still untapped by accounting historians, medieval jurists, foreign to the world of numbers and to that of commerce, who felt some embarrassment facing an efficient bookkeeping. Bookkeeping impressed these intellectuals to the point of encouraging them to justify its origin, determining its rules, defining its legal status, and establishing litigation concerning those records. The medieval legalists had just one reference model: Ancient Rome. Thus, they used Roman law as a basis for thinking accounting technique that they observed in their own time. They used both their own observations of practice and Roman law to legitimate the methods used by contemporary merchants to measure, record, and store their trade flows.