The article proposes an economic theory of customary measurement systems. The form of such systems is driven by two transaction-cost factors: minimising costs of implementation, and coordinating on shared standards. These factors combine to yield seven principles of customary measurement, supported with illustrative examples from the traditional Anglo-American, Egyptian, Greek, Roman, Chinese, and Indian measurement systems. The theory illuminates various confounding features of such systems, including ubiquitous binary patterns, frequent appearance of duodecimal ratios, and persistence of trade-specific measures.