This paper analyses the contractual forms and organisational structures employed by medieval merchant-bankers. From the perspective of the history of ideas, this paper employs the example of Datini and Medici banks, their contractual predecessors and their influence in Germany in order to show how business thought is disseminated through business practice. From an accounting history perspective, this research contributes to a better understanding of commercial and accounting practices over time. It also traces the origins of modern institutional forms, revealing how accounting and organisational tools have been used to solve both historical and contemporary problems.