Personalised guanxi are often seen as the backbone of Chinese social and commercial life. Historical evidence, however, suggests that a number of impersonal economic and social mechanisms existed that limited, altered or extended guanxi-type personal relations. At the heart of these mechanisms were contracts, governed by principles of law and putative custom. This paper examines the status of customs in Republican-era contracts, showing how “customs” were manufactured by commercial interest groups to create secular and effective contracts. The empirical basis for these findings is a case study of the Shanghai Native Bankers’ Guild (Shanghai qianye gonghui) between 1917 and 1928. Native banks (qianzhuang) carefully crafted their “customs” to fit into the rapidly evolving institutional context and the secular discourse on legal reform in early Republican China. While the Guild had to cede power to commercial arbitrators and courts, it created a shared set of norms that facilitated impersonal contracting. Despite being a relatively limited case study in scope, this paper also suggests that the trend towards secular contracting went well beyond the city limits of Republican-era Shanghai.