I argue for a new conception of practical authority based on an analysis of the relationship between authority and subject. Commands entail a demand for practical deference, which establishes a relationship of hierarchy, vulnerability, and responsibility that involves a variety of signals and commitments. In order for this relationship to be justified, the subject must be under a preexisting duty, the authority's commands must take precedence over the subject's judgment regarding fulfillment of that duty, the authority must accept the position and responsibilities of command, and the authority must be sufficiently trustworthy relative to how vulnerable the subject makes herself by deferring. This results in an instrumentalist conception of practical authority that can be favorably compared to Joseph Raz's influential service conception. The relational conception's main advantage is that it focuses on the authority as much as the subject, requiring that the authority accept responsibility for the relationship and be sufficiently trustworthy. This allows the relational conception to avoid problems that the service conception faces and illuminates institutional authority.