Morality is a normative system of guidance that figures into practical reason by telling people what to do in various situations. The problem, however, is that morality has inherent gaps that often render it inefficacious. First, it may be indeterminate due to the high level of generality in which its principles are formulated. Second, moral terms such as ‘good’ and ‘right’ may be so vague that they fail to specify the requisite behavior. And third, its subjective aspect, which is a product of personal experience, generates moral disagreement and thereby creates coordination problems that frustrate society’s collective moral aims. The objective of this article is to advance the thesis that morality must sometimes depend on law as a supplementary source of practical reason, a dependence which can be explained in terms of three essential features of law: its institutional character, its claim to authority, and its status as a second-order exclusionary reason for action. It shall then be explained how these three features enable law to make difficult decisions on behalf of individuals, define objective standards of conduct, and solve coordination problems, respectively, and in doing so, manage to fill in the gaps of morality mentioned above. Hence, it will be argued that law is also a normative system that helps people achieve their moral aims, notwithstanding the fact that it guides human behavior through a different logic and mode of operation from those of morality.