Abstract: How should we interpret legal instruments? How do we identify the law they create? Current approaches largely fall into two broad camps. The standard picture of interpretation is focused on language, using various linguistic conventions to discover a document's meaning or a drafter's intent. Those who see language as less determinate take a more skeptical view, urging judges to make interpretive choices on policy grounds. Yet both approaches neglect the most important resource available: the already applicable rules of law. Legal interpretation is neither a subfield of linguistics nor an exercise in policymaking. Rather, it is deeply shaped by preexisting legal rules. These rules tell us what legal materials to read and how to read them. Like other parts of the law, what we call "the law of interpretation" has a claim to guide the actions of judges, officials, and private interpreters -- even if it isn't ideal. We argue that legal interpretive rules are conceptually possible, normatively sensible, and actually part of our legal system. This Article thus reframes the theory of statutory and constitutional interpretation, distinguishing purely linguistic questions from legal questions to which language offers no unique answer. It also has two concrete implications of note. It provides a framework for analyzing the canons of interpretation, determining whether they are legally valid and how much authority they bear. And it helps resolve debates over constitutional "interpretation" and "construction," explaining how construction can go beyond the text but not beyond the law.