Modern law often assumes that a uniform cost-benefit formula is the proper way to determine fault in ordinary contract disputes. This chapter disputes that vision by defending the view that different standards of fault are appropriate in different contexts, in line with Roman law classifications adopted in Coggs v. Bernard in 1703. Typically, parties in gratuitous transactions should be held only to the standard of care that they bring to their own affairs. The higher objective standard of ordinary care governs in commercial transactions. That bifurcation leads to efficient searches. Persons who hold themselves out as merchants or experts warrant their ability to achieve uniform standards, while individuals who seek favors from their friends are incentivized to choose them carefully. The basic principle has surprising durability in dealing with agency, medical malpractice, occupier liability, guest statute, and frustration cases. Often the efficient analysis of fault is given only to those who do economics without really trying.