THREE CASTILIAN MANUSCRIPTS ON MERCANTILE ARITHMETIC AND THEIR PROBLEMS OF ALLOYS Date of reception: 20Ih of Aprrl, 2009 Final date «f accepiance: 28Ih of July, 2009 The aim of this work is to thruw light on tlie existence of three Castilian manuscripts on mercantile arithmetic, al1 from the Courteenih century. The first, which is moreover the most gciieral arid most complete of the three, is a inantrscript entitled Libro de Arismética, wliich is kept in the Roya1 Collegiate Churcti of San lsidoro in León (MS.46). It can be considered as the first book on mercantile arithmetic written in Castiliart and it takes back, by some 100 years, the date which had beeri used trp nntil then when speakiilg of treatises on mercantile arithmetic in the Peninsula. It conlains an ample collection of practica1 examples, 192 in ail. Of these, 23 (12°%) are o11 alloys. A second manuscript is kept in tlie National Lihrary in Madrid, in the Rare Manuscripts section. It is incomplete and does not have a title and it offers us a total of 76 problems. 13 (16%) of these are about alloys. The diirdexampleanalysed, DeArfsmetica, ispreserved inThe Spanish Academy, Ss. 155, boiind in a misccllaneous work entitled Escritos Diversos. We believe that it is incomplete, although it forms a coherent and perfedly inteUigible whole. It omjts aU general aspects and, after advising us that its aim is to illustrate how to work with fractioiis, ir begins directly with a colleaion of problems 4 8 - of which only two (4%) are on alloys. Arithmetic lor merchants, Development of trade, Learning mercantile tech- niques, Manual from the lower Middle Ages and Renaissancc. Mercatorum aritllmetica, Rerum incremeritum. De re mercatoria, Enchiridion ad Medium Aeuum inferius ac Humanitatum studium tractaii. /MAGO TE~ronis. Maoiir~ Aevo,~. 111 (2009): 171-188. ISSN 1888-3911 Por the purpose of this study, we have concentrated on three Castilian manuscripts concerning commercial arithmeiic'