During the Middle Ages merchants went along the houses with their basket with goods. They tried to barter and their job was very different from the occupations of their later colleagues. In the sixteenth century merchants stayed in their offices and sent their travelling salesmen out. Business travels became longer, merchants went overseas to different countries, they had to pay salaries, custom rights, costs of transport, assurances of goods, etc. They needed to change money in many different ways, because each city had its own money system. They visited exchange banks where moneychangers took care of their affairs. Bankers and bookkeepers were needed. Many merchants earned a lot of money and they needed carpenters, bricklayers, gold- and silversmiths and others to spend this money by building houses and filling these with luxury goods. The job of the merchant and his calculations became more and more complicated and he felt the need of a new arithmetical method, a written arithmetic that could help him to write down his big numbers and his extensive calculations. During the fifteenth and sixteenth century the new scriptural arithmetic with Hindu-Arabic numbers came into use in the Netherlands. Many arithmetic books were written in the vernacular, but in spite of that it lasted several centuries before the new arithmetic had superseded the old one completely. What are the contents of the Dutch arithmetic books of the fifteenth and sixteenth centuries? Why does it take such a long time before the new arithmetic was the one and only method? At which schools was the new arithmetic taught? These are the main problems dealt with in this article.