Chapter 2, ‘A Mathematical Culture: The Art of Setting Limits’ brings the reader directly into early modern metal mines. The birth of a vernacular culture of geometry is described, detailing the daily work of craftsmen and insisting on the materiality of measuring practices. Surveys, carried out in public during solemn ceremonies, were a keystone of mining laws. The chapter exposes a central hypothesis of this book: At the time, mathematical accuracy acquired a dual meaning. Measurements had to be precise enough to solve intricate technical problems, while at the same time respecting procedures codified in mining customs and laws. Far from being a mere tool, geometry was meant to ensure trust; it was ubiquitous and pervaded many aspects of a miner’s life. In the early years of the Protestant Reformation, Lutheran pastors actively fostered the rise of practical mathematics. Mathematical and religious rationality were equated, making subterranean geometry accurate in a third way, this time as an expression of divine will. The omnipresence of measurements, combined with their legal and religious recognition, ultimately conferred a higher status to the discipline.